Question

Name and describe two response properties that adjacent layer 4 neurons in the primary visual cortex are likely to share.

Answer 1

Neurons in layer 4 of primary visual cortex are the main target of axons arising from the lateral geniculate nucleus (LGN) of the thalamus.Based on this input, layer 4 neurons represent the first stage of visual processing in the cerebral cortex. In the cat, where the physiology of layer 4 neurons has been studied most extensively, there is a dramatic transformation from the receptive-field structure of geniculate inputs to that of layer 4 neurons.While geniculate neurons have center/surround receptive fields with circular centers (on or off) and antagonistic surrounds , their target neurons simple cells in cortical layer 4 have elongated receptive fields with adjacent on and off subregions.

While the receptive-field structure that defines the simple cell is characteristic for the majority of neurons in layer 4 of cat visual cortex.The simple cell receptive field is not universal to layer 4 in all mammals. For instance, center/surround receptive fields dominate in layer 4 of tree shrew visual cortex and establish one end of a spectrum of receptive fields encountered in layer 4C of macaque visual cortex. Even among carnivores, it remains to be determined whether the simple cell receptive field should be viewed as the rule or the exception for neurons in layer 4.

Due to its close relation to the cat, yet dramatically earlier birth, the ferret has increasingly becoming a model system for studying the early development of the visual system. Despite the widespread use of young ferrets for addressing questions about neural development, understanding of the visual physiology in the adult ferret is quite limited. In particular, we lack any knowledge of the spatial organization of layer 4 receptive fields.

Based on reports that on and off geniculate axons terminate in nonoverlapping patches within layer 4 of the ferret and layer 4 neurons with similar on versus off preferences tend to cluster in the mink, one might expect to find patches of layer 4 cells in the ferret with center/surround receptive fields of the same sign or patches with elongated single-subunit receptive fields. Contrary to this prediction, however, optical imaging studies have failed to identify functional on or off patches in supragranular cortex .Thus it remains to be determined whether or not individual layer 4 neurons in the ferret have access to information from both the on and off pathways.

A separate but related issue is whether or not layer 4 neurons in the ferret display orientation selectivity. To our knowledge, only two studies have examined orientation selectivity specifically among layer 4 neurons , and both of these studies report a broad range of responses. Because the ferret has become a model system for studying the development of orientation selectivity in particular , it is critical to determine where in the cortical circuit orientation selectivity first emerges. Many models for the development of orientation selectivity are based on correlated activity among geniculate inputs to neurons in cortical layer 4. If neurons in layer 4 of ferret visual cortex indeed display orientation selectivity, then this finding would be consistent with these correlation-based models.

Please give a like for the answer.
Thank you.