Behavioral and Neuronal Substrates of Invariant Object Recognition in Rats

The visual system of humans and other primates has the remarkable ability to recognize objects despite tremendous variation in their appearance, due to changes in size, position, background, and viewpoint. While this ability is central to human visual perception, the underlying brain mechanisms are poorly understood, and transformation-tolerant recognition remains a major challenge in the development of artificial vision systems. Arguably, this is a consequence of the formidable complexity of the primate visual system and the relatively narrow range of experimental approaches that human and nonhuman primate studies allow. Although, traditionally, the invasive study of the neuronal basis of object vision has been restricted to non-human primate experiments, recently, rodents are merging as powerful models to study visual processing. However, successful use of rodents as models for studying visual object recognition crucially depends on the ability of their visual system to construct representations of visual objects that tolerate (i.e., remain relatively unchanged with respect to) the tremendous changes in object appearance produced, for instance, by size and viewpoint variation. As the first part of this Thesis, I addressed this question by training rats to categorize a continuum of morph objects resulting from blending two object prototypes. The resulting psychometric curve (reporting the proportion of responses to one prototype along the morph line) served as a reference when, in a second phase of the experiment, either prototype was briefly presented as a prime, immediately before a test morph object. The resulting shift of the psychometric curve showed that recognition became biased (primed) toward the identity of the prime. Critically, this bias was observed also when the primes were transformed along a variety of dimensions (i.e., size, position, viewpoint, and their combination) that the animals had never experienced before. These results indicate that rats spontaneously perceive different views/appearances of an object as similar (i.e., as instances of the same object) and argue for the existence of neuronal substrates underlying formation of transformation-tolerant object representations in rats. As the next step, I tried to characterize such neuronal substrates by performing multi- electrode neuronal recordings (in anesthetized rats exposed to a battery of visual objects) from five different cortical areas of the rat brain: primary visual cortex (V1) and four extrastriate areas (named LM, AL, LI and LL) that are located laterally to V1 and have been proposed as candidate stages of a putative rat visual shape processing stream,homologous to the monkey 5 ventral visual stream (Apart from area AL that probably belongs to dorsal pathway in rat). An object set consisting of 10 different objects, each transformed across a variety of axes (position, size, in-depth azimuth rotation and in-plane rotation) was used. I found that along the processing hierarchy V1->LM->LI->LL, receptive fields become progressively bigger, as well as the latency of the response. Using information theory I found that, as the information travels through this hierarchy, the fractional information that each cell carries about the luminance gradually decreases, whereas the fractional information about shape gradually increases. Accordingly, I found that neurons along this pathway become increasingly tolerant to transformations. This indicates that neurons along this hierarchy become progressively tuned to more complex visual attributes and become more tolerant to transformations, thus suggesting that the pathway V1->LM->LI->LL could be homologous to the primate ventral stream. Overall, the combination of this behavioral and neurophysiological studies will provide an unprecedented understanding of high-level visual processing in a rodent species

Prune and distill: similar reformatting of image information along rat visual cortex and deep neural networks

Visual object recognition has been extensively studied in both neuroscience and computer vision. Recently, the most popular class of artificial systems for this task, deep convolutional neural networks (CNNs), has been shown to provide excellent models for its functional analogue in the brain, the ventral stream in visual cortex. This has prompted questions on what, if any, are the common principles underlying the reformatting of visual information as it flows through a CNN or the ventral stream. Here we consider some prominent statistical patterns that are known to exist in the internal representations of either CNNs or the visual cortex and look for them in the other system. We show that intrinsic dimensionality (ID) of object representations along the rat homologue of the ventral stream presents two distinct expansion-contraction phases, as previously shown for CNNs. Conversely, in CNNs, we show that training results in both distillation and active pruning (mirroring the increase in ID) of low- to middle-level image information in single units, as representations gain the ability to support invariant discrimination, in agreement with previous observations in rat visual cortex. Taken together, our findings suggest that CNNs and visual cortex share a similarly tight relationship between dimensionality expansion/reduction of object representations and reformatting of image information.Comment: 11 pages, 5 fiure

Data from: Emergence of transformation-tolerant representations of visual objects in rat lateral extrastriate cortex

Rodents are emerging as increasingly popular models of visual functions. Yet, evidence that rodent visual cortex is capable of advanced visual processing, such as object recognition, is limited. Here we investigate how neurons located along the progression of extrastriate areas that, in the rat brain, run laterally to primary visual cortex, encode object information. We found a progressive functional specialization of neural responses along these areas, with: (1) a sharp reduction of the amount of low-level, energy-related visual information encoded by neuronal firing; and (2) a substantial increase in the ability of both single neurons and neuronal populations to support discrimination of visual objects under identity-preserving transformations (e.g., position and size changes). These findings strongly argue for the existence of a rat object-processing pathway, and point to the rodents as promising models to dissect the neuronal circuitry underlying transformation-tolerant recognition of visual objects

Source data file for the article authored by Tafazoli and colleagues on invariant object representations in rat visual cortex

This data file provides the full data set processed in the article “Emergence of transformation-tolerant representations of visual objects in rat lateral extrastriate cortex” by Sina Tafazoli, Houman Safaai, Gioia De Franceschi, Federica Bianca Rosselli, Walter Vanzella, Margherita Riggi, Federica Buffolo, Stefano Panzeri and Davide Zoccolan. A detailed description of the file is provided in the companion "Readme.pdf" file