Distinct recruitment of feedforward and recurrent pathways across higher-order areas of mouse visual cortex.

Cortical visual processing transforms features of the external world into increasingly complex and specialized neuronal representations. These transformations arise in part through target-specific routing of information; however, within-area computations may also contribute to area-specific function. Here, we sought to determine whether higher order visual cortical areas lateromedial (LM), anterolateral (AL), posteromedial (PM), and anteromedial (AM) have specialized anatomical and physiological properties by using a combination of whole-cell recordings and optogenetic stimulation of primary visual cortex (V1) axons in vitro.

High-fidelity optical excitation of cortico-cortical projections at physiological frequencies.

Optogenetic activation of axons is a powerful approach for determining the synaptic properties and impact of long-range projections both in vivo and in vitro. However, because of the difficulty of measuring activity in axons, our knowledge of the reliability of optogenetic axonal stimulation has relied on data from somatic recordings. Yet, there are many reasons why activation of axons may not be comparable to cell bodies.

Chromatic detection from cone photoreceptors to V1 neurons to behavior in rhesus monkeys.

Chromatic sensitivity cannot exceed limits set by noise in the cone photoreceptors. To determine how close neurophysiological and psychophysical chromatic sensitivity come to these limits, we developed a parameter-free model of stimulus encoding in the cone outer segments, and we compared the sensitivity of the model to the psychophysical sensitivity of monkeys performing a detection task and to the sensitivity of individual V1 neurons. Modeled cones had a temporal impulse response and a noise power spectrum that were derived from in vitro recordings of macaque cones, and V1 recordings were made during performance of the detection task.

V1 mechanisms underlying chromatic contrast detection.

To elucidate the cortical mechanisms of color vision, we recorded from individual primary visual cortex (V1) neurons in macaque monkeys performing a chromatic detection task. Roughly 30% of the neurons that we encountered were unresponsive at the monkeys' psychophysical detection threshold (PT). The other 70% were responsive at threshold but on average, were slightly less sensitive than the monkey.

Nonlinear analysis of macaque V1 color tuning reveals cardinal directions for cortical color processing.

Understanding color vision requires knowing how signals from the three classes of cone photoreceptor are combined in the cortex. We recorded from individual neurons in the primary visual cortex (V1) of awake monkeys while an automated, closed-loop system identified stimuli that differed in cone contrast but evoked the same response. We found that isoresponse surfaces for half the neurons were planar, which is consistent with linear processing.

Effects of microsaccades on contrast detection and V1 responses in macaques.

Microsaccades can elevate contrast detection thresholds of human observers and modulate the activity of neurons in monkey visual cortex. Whether microsaccades elevate contrast detection thresholds in monkey observers is not known and bears on the interpretation of neurophysiological experiments. To answer this question, we trained two monkeys to perform a 2AFC contrast detection task.

The morphology of supragranular pyramidal neurons in the human insular cortex: a quantitative Golgi study.

Although the primate insular cortex has been studied extensively, a comprehensive investigation of its neuronal morphology has yet to be completed. To that end, neurons from 20 human subjects (10 males and 10 females; N = 600) were selected from the secondary gyrus brevis, precentral gyrus, and postcentral gyrus of the left insula. The secondary gyrus brevis was generally more complex in terms of dendritic/spine extent than either the precentral or postcentral insular gyri, which is consistent with the posterior-anterior gradient of dendritic complexity observed in other cortical regions.