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Title: How well do deep neural networks trained on object recognition characterize the mouse visual system?

Paper content:
Recent work on modeling neural responses in the primate visual system has benefited from deep neural networks trained on large-scale object recognition, and found a hierarchical correspondence between layers of the artificial neural network and brain areas along the ventral visual stream.
However, we neither know whether such task-optimized networks enable equally good models of the rodent visual system, nor if a similar hierarchical correspondence exists.
Here, we address these questions in the mouse visual system by extracting features at several layers of a convolutional neural network (CNN) trained on ImageNet to predict the responses of thousands of neurons in four visual areas (V1, LM, AL, RL) to natural images.
We found that the CNN features outperform classical subunit energy models, but found no evidence for an order of the areas we recorded via a correspondence to the hierarchy of CNN layers.
Moreover, the same CNN but with random weights provided an equivalently useful feature space for predicting neural responses.
Our results suggest that object recognition as a high-level task does not provide more discriminative features to characterize the mouse visual system than a random network.
Unlike in the primate, training on ethologically relevant visually guided behaviors -- beyond static object recognition -- may be needed to unveil the functional organization of the mouse visual cortex.
Visual object recognition is a fundamental and difficult task performed by the primate brain via a hierarchy of visual areas (the ventral stream) that progressively untangles object identity information, gaining invariance to a wide range of object-preserving visual transformations [1, 2] .
Fueled by the advances of deep learning, recent work on modeling neural responses in sensory brain areas builds upon hierarchical convolutional neural networks (CNNs) trained to solve complex tasks like object recognition [3] .
Interestingly, these models have not only achieved unprecedented performance in predicting neural responses in several brain areas of macaques and humans [4] [5] [6] [7] , but they also revealed a hierarchical correspondence between the layers of the CNNs and areas of the ventral stream [4, 6] : the higher the area in the ventral stream, the higher the CNN layer that explained it best.
The same approach also provided a quantitative signature of a previously unclear hierarchical organization of A1 and A2 in the human auditory cortex [7] .
These discoveries about the primate have sparked a still unresolved question: to what extent is visual object processing also hierarchically organized in the mouse visual cortex and how well can the mouse visual system be modeled using goal-driven deep neural networks trained on static object classification?
This question is important since mice are increasingly used to study vision due to the plethora of available experimental techniques such as the ability to genetically identify and manipulate neural circuits that are not easily available in primates.
Recent work suggests that rats are capable of complex visual discrimination tasks [8] and recordings from extrastriate areas show a gradual increase in the ability of neurons in higher visual areas to support discrimination of visual objects [9, 10] .
Here, we set out to study how well the mouse visual system can be characterized by goal-driven deep neural networks.
We extracted features from the hidden layers of a standard CNN (VGG16, [11] ) trained on object categorization, to predict responses of thousands of neurons in four mouse visual areas (V1, LM, AL, RL) to static natural images.
We found that VGG16 yields powerful features for predicting neural activity, outperforming a Gabor filter bank energy model in these four visual areas.
However, VGG16 does not significantly outperform a feature space produced by a network with an identical architecture but random weights.
In contrast to previous work in primates, our data provide no evidence so far for a hierarchical correspondence between the deep network layers and the visual areas we recorded.
trough the core (A) network (first n layers of VGG16) to produce a feature space shared by all neurons.
Then, the spatial transformer readout (B) finds a mapping between these features and the neural responses for each neuron separately.
The shifter network (an MLP with one hidden layer) corrects for eye movements.
The output of the readout is multiplied by a gain predicted by the modulator network (an MLP with one hidden layer) that uses running speed and pupil dilation.
A static nonlinearity converts the result into the predicted spike rate.
All components of the model are trained jointly end-to-end to minimize the difference between predicted and observed neural responses.
In contrast to similar work in the primate, we find no match between the hierarchy of mouse visual cortical areas and the layers of CNNs trained on object categorization.
Although VGG16 achieves state-of-the-art performance, it is matched by random weights.
There are three implications of our results: First, our work is in line with previous work in machine learning that shows the power of random features [15] .
Therefore, we argue that models based on random features should always be reported as baselines in studies on neural system identification.
Second, which VGG layer best predicted any given brain area depended strongly on the image resolution we used to feed into VGG16.
We observed a similar effect in our earlier work on primate V1 [5] .
Thus, the studies reporting a hierarchical correspondence between goal-driven deep neural networks and the primate ventral stream should be taken with a grain of salt, as they -to the best of our knowledge -do not include this control.
Third, optimizing the network for static object recognition alone as a high-level goal does not appear to be the right approximation to describe representations and the visual hierarchy in the mouse cortex.
Although our results do not exclude a potential object processing hierarchy in the mouse visual system, they suggest that training with more ethologically relevant visually guided tasks for the mouse could be a more fruitful goal-driven approach to characterize the mouse visual system [16] .
For instance, an approach with dynamic stimuli such as those found during prey capture tasks [17] could yield more meaningful features to unveil the functional organization of the mouse visual system.

assistant: text: A goal-driven approach to model four mouse visual areas (V1, LM, AL, RL) based on deep neural networks trained on static object recognition does not unveil a functional organization of visual cortex unlike in primates