|Title||Increases in selectivity are offset by increases in tolerance ("invariance") to maintain sparseness across the ventral visual pathway|
|Publication Type||Conference Proceedings|
|Year of Publication||2008|
|Authors||Rust, NC, DiCarlo, JJ|
|Conference Name||Society for Neuroscience|
|Conference Location||Washington, DC, USA|
Popular accounts of visual processing suggest that neurons become increasingly selective for particular objects and scenes as signals propagate through the ventral visual pathway, but this hypothesis has proven difficult to test. To investigate this issue systematically, we recorded the responses of neurons in a mid-level visual area (V4) and a high-level visual area (IT) under well-controlled conditions (same monkeys, same task, same region of the visual field, same stimuli). We assessed the selectivity of neurons in each area by determining how well each population could discriminate between natural images and “scrambled” versions of those images that have the same local structure but configured randomly. We found that the V4 population discriminated between members of the two image sets with similar fidelity whereas discrimination by the IT population was considerably degraded for the scrambled as compared to the natural images. These results suggest that IT neurons are more selective than V4 neurons in terms of the image features that drive these cells. As a second estimate of selectivity, we measured the tuning bandwidth of neurons for natural images (commonly called “sparseness”). Surprisingly, we found that distributions of sparseness values were indistinguishable between V4 and IT. Similarly, we found that individual images activated the same fraction of neurons in V4 and IT, suggesting that a coding principle is conserved at each level of processing. How can the selectivity for natural image features increase while the tuning bandwidth for natural images remains constant? One possible explanation is that increases in selectivity for particular image features are offset by increases in tolerance for the (e.g.) position and scale of those features. Indeed, when we measured the tolerance of neurons to changes in the position and scale of images, we found that tolerance increases as signals propagate from V4 to IT. Moreover, we found that equivalent sparseness values were correlated with higher levels of selectivity and tolerance in IT as compared to V4. These results confirm that neurons increase both their selectivity for image features and their tolerance to changes in the position and scale of those features as signals propagate through the ventral visual pathway. Remarkably, the rates of increase of these two parameters appear to be set such that an equally distributed coding scheme is maintained at each level of visual processing.