diff --git a/.github/PULL_REQUEST_TEMPLATE.md b/.github/PULL_REQUEST_TEMPLATE.md
index 029220c..e1c3f2b 100644
--- a/.github/PULL_REQUEST_TEMPLATE.md
+++ b/.github/PULL_REQUEST_TEMPLATE.md
@@ -1,6 +1,6 @@
 - [ ] I have added the correct label(s) to this Pull Request or linked the relevant issue(s)
 - [ ] I have provided a description of the changes in this Pull Request
-- [ ] I have added documentation for my changes
+- [ ] I have added documentation for my changes and have listed relevant changes in CHANGELOG.md
 - [ ] If applicable, I have added tests to cover my changes.
 - [ ] I have reformatted the code using `poe format` 
 - [ ] I have checked style and types with `poe lint` and `poe type-check`
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9dd8459..33622ce 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,11 +19,24 @@
 - New `evaluation` package for repeating the same experiment with multiple seeds and aggregating the results (important extension!). 
 Launchers for parallelization currently in alpha state. #1074
 - Loggers can now restore the logged data into python by using the new `restore_logged_data` method. #1074
+- `continuous.Critic`:
+  - Add flag `apply_preprocess_net_to_obs_only` to allow the
+    preprocessing network to be applied to the observations only (without
+    the actions concatenated), which is essential for the case where we want
+    to reuse the actor's preprocessing network #1128
 - Base class for collectors: `BaseCollector` #1122
 - Collectors can now explicitly specify whether to use the policy in training or evaluation mode. #1122
 - New util context managers `in_eval_mode` and `in_train_mode` for torch modules. #1122
 - `reset` of `Collectors` now returns `obs` and `info`. #1122
 
+### Fixes
+- `CriticFactoryReuseActor`: Enable the Critic flag `apply_preprocess_net_to_obs_only` for continuous critics, 
+  fixing the case where we want to reuse an actor's preprocessing network for the critic (affects usages
+  of the experiment builder method `with_critic_factory_use_actor` with continuous environments) #1128
+- `atari_network.DQN`:
+  - Fix constructor input validation #1128
+  - Fix `output_dim` not being set if `features_only`=True and `output_dim_added_layer` is not None #1128
+
 ### Internal Improvements
 - `Collector`s rely less on state, the few stateful things are stored explicitly instead of through a `.data` attribute. #1063
 - Introduced a first iteration of a naming convention for vars in `Collector`s. #1063
diff --git a/docs/02_notebooks/L7_Experiment.ipynb b/docs/02_notebooks/L7_Experiment.ipynb
index 9a97b20..0a6675c 100644
--- a/docs/02_notebooks/L7_Experiment.ipynb
+++ b/docs/02_notebooks/L7_Experiment.ipynb
@@ -152,7 +152,7 @@
     "id": "Lh2-hwE5Dn9I"
    },
    "source": [
-    "Once we have defined the actor, the critic and the optimizer. We can use them to construct our PPO agent. CartPole is a discrete action space problem, so the distribution of our action space can be a categorical distribution."
+    "Once we have defined the actor, the critic and the optimizer, we can use them to construct our PPO agent. CartPole is a discrete action space problem, so the distribution of our action space can be a categorical distribution."
    ]
   },
   {
diff --git a/docs/spelling_wordlist.txt b/docs/spelling_wordlist.txt
index 383429e..83de823 100644
--- a/docs/spelling_wordlist.txt
+++ b/docs/spelling_wordlist.txt
@@ -266,3 +266,8 @@ postfix
 backend
 rliable
 hl
+v_s
+v_s_
+obs
+obs_next
+
diff --git a/examples/atari/atari_network.py b/examples/atari/atari_network.py
index ea900e9..4f2a560 100644
--- a/examples/atari/atari_network.py
+++ b/examples/atari/atari_network.py
@@ -66,7 +66,7 @@ class DQN(NetBase[Any]):
         layer_init: Callable[[nn.Module], nn.Module] = lambda x: x,
     ) -> None:
         # TODO: Add docstring
-        if features_only and output_dim_added_layer is not None:
+        if not features_only and output_dim_added_layer is not None:
             raise ValueError(
                 "Should not provide explicit output dimension using `output_dim_added_layer` when `features_only` is true.",
             )
@@ -98,6 +98,7 @@ class DQN(NetBase[Any]):
                 layer_init(nn.Linear(base_cnn_output_dim, output_dim_added_layer)),
                 nn.ReLU(inplace=True),
             )
+            self.output_dim = output_dim_added_layer
         else:
             self.output_dim = base_cnn_output_dim
 
diff --git a/poetry.lock b/poetry.lock
index 2a9fe13..fb041f1 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -1,4 +1,4 @@
-# This file is automatically @generated by Poetry 1.7.1 and should not be changed by hand.
+# This file is automatically @generated by Poetry 1.8.2 and should not be changed by hand.
 
 [[package]]
 name = "absl-py"
@@ -156,7 +156,7 @@ files = [
 name = "arch"
 version = "5.3.1"
 description = "ARCH for Python"
-optional = true
+optional = false
 python-versions = ">=3.7"
 files = [
     {file = "arch-5.3.1-cp310-cp310-macosx_10_9_x86_64.whl", hash = "sha256:75fa6f9386ecc2df81bcbf5d055a290a697482ca51e0b3459dab183d288993cb"},
@@ -3815,75 +3815,6 @@ sql-other = ["SQLAlchemy (>=1.4.36)"]
 test = ["hypothesis (>=6.46.1)", "pytest (>=7.3.2)", "pytest-asyncio (>=0.17.0)", "pytest-xdist (>=2.2.0)"]
 xml = ["lxml (>=4.8.0)"]
 
-[[package]]
-name = "pandas"
-version = "2.2.2"
-description = "Powerful data structures for data analysis, time series, and statistics"
-optional = false
-python-versions = ">=3.9"
-files = [
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-    {file = "pandas-2.2.2-cp310-cp310-macosx_11_0_arm64.whl", hash = "sha256:c7adfc142dac335d8c1e0dcbd37eb8617eac386596eb9e1a1b77791cf2498238"},
-    {file = "pandas-2.2.2-cp310-cp310-manylinux_2_17_aarch64.manylinux2014_aarch64.whl", hash = "sha256:4abfe0be0d7221be4f12552995e58723c7422c80a659da13ca382697de830c08"},
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-    {file = "pandas-2.2.2.tar.gz", hash = "sha256:9e79019aba43cb4fda9e4d983f8e88ca0373adbb697ae9c6c43093218de28b54"},
-]
-
-[package.dependencies]
-numpy = {version = ">=1.23.2", markers = "python_version == \"3.11\""}
-python-dateutil = ">=2.8.2"
-pytz = ">=2020.1"
-tzdata = ">=2022.7"
-
-[package.extras]
-all = ["PyQt5 (>=5.15.9)", "SQLAlchemy (>=2.0.0)", "adbc-driver-postgresql (>=0.8.0)", "adbc-driver-sqlite (>=0.8.0)", "beautifulsoup4 (>=4.11.2)", "bottleneck (>=1.3.6)", "dataframe-api-compat (>=0.1.7)", "fastparquet (>=2022.12.0)", "fsspec (>=2022.11.0)", "gcsfs (>=2022.11.0)", "html5lib (>=1.1)", "hypothesis (>=6.46.1)", "jinja2 (>=3.1.2)", "lxml (>=4.9.2)", "matplotlib (>=3.6.3)", "numba (>=0.56.4)", "numexpr (>=2.8.4)", "odfpy (>=1.4.1)", "openpyxl (>=3.1.0)", "pandas-gbq (>=0.19.0)", "psycopg2 (>=2.9.6)", "pyarrow (>=10.0.1)", "pymysql (>=1.0.2)", "pyreadstat (>=1.2.0)", "pytest (>=7.3.2)", "pytest-xdist (>=2.2.0)", "python-calamine (>=0.1.7)", "pyxlsb (>=1.0.10)", "qtpy (>=2.3.0)", "s3fs (>=2022.11.0)", "scipy (>=1.10.0)", "tables (>=3.8.0)", "tabulate (>=0.9.0)", "xarray (>=2022.12.0)", "xlrd (>=2.0.1)", "xlsxwriter (>=3.0.5)", "zstandard (>=0.19.0)"]
-aws = ["s3fs (>=2022.11.0)"]
-clipboard = ["PyQt5 (>=5.15.9)", "qtpy (>=2.3.0)"]
-compression = ["zstandard (>=0.19.0)"]
-computation = ["scipy (>=1.10.0)", "xarray (>=2022.12.0)"]
-consortium-standard = ["dataframe-api-compat (>=0.1.7)"]
-excel = ["odfpy (>=1.4.1)", "openpyxl (>=3.1.0)", "python-calamine (>=0.1.7)", "pyxlsb (>=1.0.10)", "xlrd (>=2.0.1)", "xlsxwriter (>=3.0.5)"]
-feather = ["pyarrow (>=10.0.1)"]
-fss = ["fsspec (>=2022.11.0)"]
-gcp = ["gcsfs (>=2022.11.0)", "pandas-gbq (>=0.19.0)"]
-hdf5 = ["tables (>=3.8.0)"]
-html = ["beautifulsoup4 (>=4.11.2)", "html5lib (>=1.1)", "lxml (>=4.9.2)"]
-mysql = ["SQLAlchemy (>=2.0.0)", "pymysql (>=1.0.2)"]
-output-formatting = ["jinja2 (>=3.1.2)", "tabulate (>=0.9.0)"]
-parquet = ["pyarrow (>=10.0.1)"]
-performance = ["bottleneck (>=1.3.6)", "numba (>=0.56.4)", "numexpr (>=2.8.4)"]
-plot = ["matplotlib (>=3.6.3)"]
-postgresql = ["SQLAlchemy (>=2.0.0)", "adbc-driver-postgresql (>=0.8.0)", "psycopg2 (>=2.9.6)"]
-pyarrow = ["pyarrow (>=10.0.1)"]
-spss = ["pyreadstat (>=1.2.0)"]
-sql-other = ["SQLAlchemy (>=2.0.0)", "adbc-driver-postgresql (>=0.8.0)", "adbc-driver-sqlite (>=0.8.0)"]
-test = ["hypothesis (>=6.46.1)", "pytest (>=7.3.2)", "pytest-xdist (>=2.2.0)"]
-xml = ["lxml (>=4.9.2)"]
-
 [[package]]
 name = "pandocfilters"
 version = "1.5.0"
@@ -3946,7 +3877,7 @@ files = [
 name = "patsy"
 version = "0.5.6"
 description = "A Python package for describing statistical models and for building design matrices."
-optional = true
+optional = false
 python-versions = "*"
 files = [
     {file = "patsy-0.5.6-py2.py3-none-any.whl", hash = "sha256:19056886fd8fa71863fa32f0eb090267f21fb74be00f19f5c70b2e9d76c883c6"},
@@ -4213,7 +4144,7 @@ wcwidth = "*"
 name = "property-cached"
 version = "1.6.4"
 description = "A decorator for caching properties in classes (forked from cached-property)."
-optional = true
+optional = false
 python-versions = ">= 3.5"
 files = [
     {file = "property-cached-1.6.4.zip", hash = "sha256:3e9c4ef1ed3653909147510481d7df62a3cfb483461a6986a6f1dcd09b2ebb73"},
@@ -5077,7 +5008,7 @@ files = [
 name = "rliable"
 version = "1.0.8"
 description = "rliable: Reliable evaluation on reinforcement learning and machine learning benchmarks."
-optional = true
+optional = false
 python-versions = "*"
 files = []
 develop = false
@@ -5366,7 +5297,7 @@ test = ["asv", "gmpy2", "mpmath", "pooch", "pytest", "pytest-cov", "pytest-timeo
 name = "seaborn"
 version = "0.13.2"
 description = "Statistical data visualization"
-optional = true
+optional = false
 python-versions = ">=3.8"
 files = [
     {file = "seaborn-0.13.2-py3-none-any.whl", hash = "sha256:636f8336facf092165e27924f223d3c62ca560b1f2bb5dff7ab7fad265361987"},
@@ -6214,7 +6145,7 @@ tests = ["cython", "littleutils", "pygments", "pytest", "typeguard"]
 name = "statsmodels"
 version = "0.14.0"
 description = "Statistical computations and models for Python"
-optional = true
+optional = false
 python-versions = ">=3.8"
 files = [
     {file = "statsmodels-0.14.0-cp310-cp310-macosx_10_9_x86_64.whl", hash = "sha256:16bfe0c96a53b20fa19067e3b6bd2f1d39e30d4891ea0d7bc20734a0ae95942d"},
@@ -6260,51 +6191,6 @@ build = ["cython (>=0.29.26)"]
 develop = ["colorama", "cython (>=0.29.26)", "cython (>=0.29.28,<3.0.0)", "flake8", "isort", "joblib", "matplotlib (>=3)", "oldest-supported-numpy (>=2022.4.18)", "pytest (>=7.0.1,<7.1.0)", "pytest-randomly", "pytest-xdist", "pywinpty", "setuptools-scm[toml] (>=7.0.0,<7.1.0)"]
 docs = ["ipykernel", "jupyter-client", "matplotlib", "nbconvert", "nbformat", "numpydoc", "pandas-datareader", "sphinx"]
 
-[[package]]
-name = "statsmodels"
-version = "0.14.2"
-description = "Statistical computations and models for Python"
-optional = true
-python-versions = ">=3.9"
-files = [
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-    {file = "statsmodels-0.14.2.tar.gz", hash = "sha256:890550147ad3a81cda24f0ba1a5c4021adc1601080bd00e191ae7cd6feecd6ad"},
-]
-
-[package.dependencies]
-numpy = ">=1.22.3"
-packaging = ">=21.3"
-pandas = ">=1.4,<2.1.0 || >2.1.0"
-patsy = ">=0.5.6"
-scipy = ">=1.8,<1.9.2 || >1.9.2"
-
-[package.extras]
-build = ["cython (>=0.29.33)"]
-develop = ["colorama", "cython (>=0.29.33)", "cython (>=3.0.10,<4)", "flake8", "isort", "joblib", "matplotlib (>=3)", "pytest (>=7.3.0,<8)", "pytest-cov", "pytest-randomly", "pytest-xdist", "pywinpty", "setuptools-scm[toml] (>=8.0,<9.0)"]
-docs = ["ipykernel", "jupyter-client", "matplotlib", "nbconvert", "nbformat", "numpydoc", "pandas-datareader", "sphinx"]
-
 [[package]]
 name = "swig"
 version = "4.2.0"
@@ -6530,13 +6416,13 @@ files = [
 
 [[package]]
 name = "tqdm"
-version = "4.66.1"
+version = "4.66.3"
 description = "Fast, Extensible Progress Meter"
 optional = false
 python-versions = ">=3.7"
 files = [
-    {file = "tqdm-4.66.1-py3-none-any.whl", hash = "sha256:d302b3c5b53d47bce91fea46679d9c3c6508cf6332229aa1e7d8653723793386"},
-    {file = "tqdm-4.66.1.tar.gz", hash = "sha256:d88e651f9db8d8551a62556d3cff9e3034274ca5d66e93197cf2490e2dcb69c7"},
+    {file = "tqdm-4.66.3-py3-none-any.whl", hash = "sha256:4f41d54107ff9a223dca80b53efe4fb654c67efaba7f47bada3ee9d50e05bd53"},
+    {file = "tqdm-4.66.3.tar.gz", hash = "sha256:23097a41eba115ba99ecae40d06444c15d1c0c698d527a01c6c8bd1c5d0647e5"},
 ]
 
 [package.dependencies]
diff --git a/tianshou/data/buffer/base.py b/tianshou/data/buffer/base.py
index a34964f..b1719be 100644
--- a/tianshou/data/buffer/base.py
+++ b/tianshou/data/buffer/base.py
@@ -172,18 +172,19 @@ class ReplayBuffer:
         return np.array([last] if not self.done[last] and self._size else [], int)
 
     def prev(self, index: int | np.ndarray) -> np.ndarray:
-        """Return the index of previous transition.
-
-        The index won't be modified if it is the beginning of an episode.
+        """Return the index of preceding step within the same episode if it exists.
+        If it does not exist (because it is the first index within the episode),
+        the index remains unmodified.
         """
-        index = (index - 1) % self._size
+        index = (index - 1) % self._size  # compute preceding index with wrap-around
+        # end_flag will be 1 if the previous index is the last step of an episode or
+        # if it is the very last index of the buffer (wrap-around case), and 0 otherwise
         end_flag = self.done[index] | (index == self.last_index[0])
         return (index + end_flag) % self._size
 
     def next(self, index: int | np.ndarray) -> np.ndarray:
-        """Return the index of next transition.
-
-        The index won't be modified if it is the end of an episode.
+        """Return the index of next step if there is a next step within the episode.
+        If there isn't a next step, the index remains unmodified.
         """
         end_flag = self.done[index] | (index == self.last_index[0])
         return (index + (1 - end_flag)) % self._size
diff --git a/tianshou/highlevel/config.py b/tianshou/highlevel/config.py
index aa4f53e..f8ca0c0 100644
--- a/tianshou/highlevel/config.py
+++ b/tianshou/highlevel/config.py
@@ -118,9 +118,12 @@ class SamplingConfig(ToStringMixin):
     replay_buffer_ignore_obs_next: bool = False
 
     replay_buffer_save_only_last_obs: bool = False
-    """if True, only the most recent frame is saved when appending to experiences rather than the
-    full stacked frames. This avoids duplicating observations in buffer memory. Set to False to
-    save stacked frames in full.
+    """if True, for the case where the environment outputs stacked frames (e.g. because it
+    is using a `FrameStack` wrapper), save only the most recent frame so as not to duplicate
+    observations in buffer memory. Specifically, if the environment outputs observations `obs` with
+    shape (N, ...), only obs[-1] of shape (...) will be stored.
+    Frame stacking with a fixed number of frames can then be recreated at the buffer level by setting
+    :attr:`replay_buffer_stack_num`.
     """
 
     replay_buffer_stack_num: int = 1
@@ -128,6 +131,9 @@ class SamplingConfig(ToStringMixin):
     the number of consecutive environment observations to stack and use as the observation input
     to the agent for each time step. Setting this to a value greater than 1 can help agents learn
     temporal aspects (e.g. velocities of moving objects for which only positions are observed).
+
+    If the environment already stacks frames (e.g. using a `FrameStack` wrapper), this should either not
+    be used or should be used in conjunction with :attr:`replay_buffer_save_only_last_obs`.
     """
 
     @property
diff --git a/tianshou/highlevel/module/critic.py b/tianshou/highlevel/module/critic.py
index f1984e4..4eacef1 100644
--- a/tianshou/highlevel/module/critic.py
+++ b/tianshou/highlevel/module/critic.py
@@ -197,7 +197,11 @@ class CriticFactoryReuseActor(CriticFactory):
                 last_size=last_size,
             ).to(device)
         elif envs.get_type().is_continuous():
-            return continuous.Critic(actor.get_preprocess_net(), device=device).to(device)
+            return continuous.Critic(
+                actor.get_preprocess_net(),
+                device=device,
+                apply_preprocess_net_to_obs_only=True,
+            ).to(device)
         else:
             raise ValueError
 
diff --git a/tianshou/policy/base.py b/tianshou/policy/base.py
index 8753af6..bee9f9b 100644
--- a/tianshou/policy/base.py
+++ b/tianshou/policy/base.py
@@ -323,9 +323,9 @@ class BasePolicy(nn.Module, Generic[TTrainingStats], ABC):
 
         :return: A :class:`~tianshou.data.Batch` which MUST have the following keys:
 
-            * ``act`` an numpy.ndarray or a torch.Tensor, the action over \
+            * ``act`` a numpy.ndarray or a torch.Tensor, the action over \
                 given batch data.
-            * ``state`` a dict, an numpy.ndarray or a torch.Tensor, the \
+            * ``state`` a dict, a numpy.ndarray or a torch.Tensor, the \
                 internal state of the policy, ``None`` as default.
 
         Other keys are user-defined. It depends on the algorithm. For example,
@@ -587,19 +587,23 @@ class BasePolicy(nn.Module, Generic[TTrainingStats], ABC):
         advantage + value, which is exactly equivalent to using :math:`TD(\lambda)`
         for estimating returns.
 
+        Setting `v_s_` and `v_s` to None (or all zeros) and `gae_lambda` to 1.0 calculates the
+        discounted return-to-go/ Monte-Carlo return.
+
         :param batch: a data batch which contains several episodes of data in
             sequential order. Mind that the end of each finished episode of batch
             should be marked by done flag, unfinished (or collecting) episodes will be
             recognized by buffer.unfinished_index().
         :param buffer: the corresponding replay buffer.
-        :param numpy.ndarray indices: tell batch's location in buffer, batch is equal
+        :param indices: tells the batch's location in buffer, batch is equal
             to buffer[indices].
-        :param np.ndarray v_s_: the value function of all next states :math:`V(s')`.
+        :param v_s_: the value function of all next states :math:`V(s')`.
             If None, it will be set to an array of 0.
-        :param v_s: the value function of all current states :math:`V(s)`.
-        :param gamma: the discount factor, should be in [0, 1]. Default to 0.99.
+        :param v_s: the value function of all current states :math:`V(s)`. If None,
+            it is set based upon `v_s_` rolled by 1.
+        :param gamma: the discount factor, should be in [0, 1].
         :param gae_lambda: the parameter for Generalized Advantage Estimation,
-            should be in [0, 1]. Default to 0.95.
+            should be in [0, 1].
 
         :return: two numpy arrays (returns, advantage) with each shape (bsz, ).
         """
@@ -643,10 +647,10 @@ class BasePolicy(nn.Module, Generic[TTrainingStats], ABC):
         :param indices: tell batch's location in buffer
         :param function target_q_fn: a function which compute target Q value
             of "obs_next" given data buffer and wanted indices.
-        :param gamma: the discount factor, should be in [0, 1]. Default to 0.99.
+        :param gamma: the discount factor, should be in [0, 1].
         :param n_step: the number of estimation step, should be an int greater
-            than 0. Default to 1.
-        :param rew_norm: normalize the reward to Normal(0, 1), Default to False.
+            than 0.
+        :param rew_norm: normalize the reward to Normal(0, 1).
             TODO: passing True is not supported and will cause an error!
         :return: a Batch. The result will be stored in batch.returns as a
             torch.Tensor with the same shape as target_q_fn's return tensor.
diff --git a/tianshou/utils/net/continuous.py b/tianshou/utils/net/continuous.py
index 6cd4a0f..0b28f98 100644
--- a/tianshou/utils/net/continuous.py
+++ b/tianshou/utils/net/continuous.py
@@ -15,6 +15,7 @@ from tianshou.utils.net.common import (
     TLinearLayer,
     get_output_dim,
 )
+from tianshou.utils.pickle import setstate
 
 SIGMA_MIN = -20
 SIGMA_MAX = 2
@@ -109,6 +110,9 @@ class Critic(CriticBase):
         `preprocess_net`. Only used when `preprocess_net` does not have the attribute `output_dim`.
     :param linear_layer: use this module as linear layer.
     :param flatten_input: whether to flatten input data for the last layer.
+    :param apply_preprocess_net_to_obs_only: whether to apply `preprocess_net` to the observations only (before
+        concatenating with the action) - and without the observations being modified in any way beforehand.
+        This allows the actor's preprocessing network to be reused for the critic.
 
     For advanced usage (how to customize the network), please refer to
     :ref:`build_the_network`.
@@ -122,11 +126,13 @@ class Critic(CriticBase):
         preprocess_net_output_dim: int | None = None,
         linear_layer: TLinearLayer = nn.Linear,
         flatten_input: bool = True,
+        apply_preprocess_net_to_obs_only: bool = False,
     ) -> None:
         super().__init__()
         self.device = device
         self.preprocess = preprocess_net
         self.output_dim = 1
+        self.apply_preprocess_net_to_obs_only = apply_preprocess_net_to_obs_only
         input_dim = get_output_dim(preprocess_net, preprocess_net_output_dim)
         self.last = MLP(
             input_dim,
@@ -137,6 +143,14 @@ class Critic(CriticBase):
             flatten_input=flatten_input,
         )
 
+    def __setstate__(self, state: dict) -> None:
+        setstate(
+            Critic,
+            self,
+            state,
+            new_default_properties={"apply_preprocess_net_to_obs_only": False},
+        )
+
     def forward(
         self,
         obs: np.ndarray | torch.Tensor,
@@ -148,7 +162,10 @@ class Critic(CriticBase):
             obs,
             device=self.device,
             dtype=torch.float32,
-        ).flatten(1)
+        )
+        if self.apply_preprocess_net_to_obs_only:
+            obs, _ = self.preprocess(obs)
+        obs = obs.flatten(1)
         if act is not None:
             act = torch.as_tensor(
                 act,
@@ -156,8 +173,9 @@ class Critic(CriticBase):
                 dtype=torch.float32,
             ).flatten(1)
             obs = torch.cat([obs, act], dim=1)
-        values_B, hidden_BH = self.preprocess(obs)
-        return self.last(values_B)
+        if not self.apply_preprocess_net_to_obs_only:
+            obs, _ = self.preprocess(obs)
+        return self.last(obs)
 
 
 class ActorProb(BaseActor):
diff --git a/tianshou/utils/pickle.py b/tianshou/utils/pickle.py
new file mode 100644
index 0000000..9247162
--- /dev/null
+++ b/tianshou/utils/pickle.py
@@ -0,0 +1,97 @@
+"""Helper functions for persistence/pickling, which have been copied from sensAI (specifically `sensai.util.pickle`)."""
+
+from collections.abc import Iterable
+from copy import copy
+from typing import Any
+
+
+def setstate(
+    cls: type,
+    obj: Any,
+    state: dict[str, Any],
+    renamed_properties: dict[str, str] | None = None,
+    new_optional_properties: list[str] | None = None,
+    new_default_properties: dict[str, Any] | None = None,
+    removed_properties: list[str] | None = None,
+) -> None:
+    """Helper function for safe implementations of `__setstate__` in classes, which appropriately handles the cases where
+    a parent class already implements `__setstate__` and where it does not. Call this function whenever you would actually
+    like to call the super-class' implementation.
+    Unfortunately, `__setstate__` is not implemented in `object`, rendering `super().__setstate__(state)` invalid in the general case.
+
+    :param cls: the class in which you are implementing `__setstate__`
+    :param obj: the instance of `cls`
+    :param state: the state dictionary
+    :param renamed_properties: a mapping from old property names to new property names
+    :param new_optional_properties: a list of names of new property names, which, if not present, shall be initialized with None
+    :param new_default_properties: a dictionary mapping property names to their default values, which shall be added if they are not present
+    :param removed_properties: a list of names of properties that are no longer being used
+    """
+    # handle new/changed properties
+    if renamed_properties is not None:
+        for mOld, mNew in renamed_properties.items():
+            if mOld in state:
+                state[mNew] = state[mOld]
+                del state[mOld]
+    if new_optional_properties is not None:
+        for mNew in new_optional_properties:
+            if mNew not in state:
+                state[mNew] = None
+    if new_default_properties is not None:
+        for mNew, mValue in new_default_properties.items():
+            if mNew not in state:
+                state[mNew] = mValue
+    if removed_properties is not None:
+        for p in removed_properties:
+            if p in state:
+                del state[p]
+    # call super implementation, if any
+    s = super(cls, obj)
+    if hasattr(s, "__setstate__"):
+        s.__setstate__(state)
+    else:
+        obj.__dict__ = state
+
+
+def getstate(
+    cls: type,
+    obj: Any,
+    transient_properties: Iterable[str] | None = None,
+    excluded_properties: Iterable[str] | None = None,
+    override_properties: dict[str, Any] | None = None,
+    excluded_default_properties: dict[str, Any] | None = None,
+) -> dict[str, Any]:
+    """Helper function for safe implementations of `__getstate__` in classes, which appropriately handles the cases where
+    a parent class already implements `__getstate__` and where it does not. Call this function whenever you would actually
+    like to call the super-class' implementation.
+    Unfortunately, `__getstate__` is not implemented in `object`, rendering `super().__getstate__()` invalid in the general case.
+
+    :param cls: the class in which you are implementing `__getstate__`
+    :param obj: the instance of `cls`
+    :param transient_properties: transient properties which shall be set to None in serializations
+    :param excluded_properties: properties which shall be completely removed from serializations
+    :param override_properties: a mapping from property names to values specifying (new or existing) properties which are to be set;
+        use this to set a fixed value for an existing property or to add a completely new property
+    :param excluded_default_properties: properties which shall be completely removed from serializations, if they are set
+        to the given default value
+    :return: the state dictionary, which may be modified by the receiver
+    """
+    s = super(cls, obj)
+    d = s.__getstate__() if hasattr(s, "__getstate__") else obj.__dict__
+    d = copy(d)
+    if transient_properties is not None:
+        for p in transient_properties:
+            if p in d:
+                d[p] = None
+    if excluded_properties is not None:
+        for p in excluded_properties:
+            if p in d:
+                del d[p]
+    if override_properties is not None:
+        for k, v in override_properties.items():
+            d[k] = v
+    if excluded_default_properties is not None:
+        for p, v in excluded_default_properties.items():
+            if p in d and d[p] == v:
+                del d[p]
+    return d