The firing rates of neurons in primary visual cortex (V1) are suppressed by large stimuli, an effect known as surround suppression. In cats and monkeys, the strength of suppression is sensitive to orientation; responses to regions containing uniform orientations are more suppressed than those containing orientation contrast. This effect is thought to be important for scene segmentation, but the underlying neural mechanisms are poorly understood. We asked whether it is possible to study these mechanisms in the visual cortex of mice, because of recent advances in technology for studying the cortical circuitry in mice. It is unknown whether neurons in mouse V1 are sensitive to orientation contrast. We measured the orientation selectivity of surround suppression in the different layers of mouse V1. We found strong surround suppression in layer 4 and the superficial layers, part of which was orientation tuned: iso-oriented surrounds caused more suppression than cross-oriented surrounds. Surround suppression was delayed relative to the visual response and orientation-tuned suppression was delayed further, suggesting two separate suppressive mechanisms. Previous studies proposed that surround suppression depends on the activity of inhibitory somatostatin-positive interneurons in the superficial layers. To test the involvement of the superficial layers we topically applied lidocaine. Silencing of the superficial layers did not prevent orientation-tuned suppression in layer 4. These results show that neurons in mouse V1, which lacks orientation columns, show orientation-dependent surround suppression in layer 4 and the superficial layers and that surround suppression in layer 4 does not require contributions from neurons in the superficial layers.

Neurons in the rostral superior temporal sulcus (STS) are responsive to displays of body movements. We employed a parametric action space to determine how similarities among actions are represented by visual temporal neurons and how form and motion information contributes to their responses. The stimulus space consisted of a stick-plus-point-light figure performing arm actions and their blends. Multidimensional scaling showed that the responses of temporal neurons represented the ordinal similarity between these actions. Further tests distinguished neurons responding equally strongly to static presentations and to actions ("snapshot" neurons), from those responding much less strongly to static presentations, but responding well when motion was present ("motion" neurons). The "motion" neurons were predominantly found in the upper bank/fundus of the STS, and "snapshot" neurons in the lower bank of the STS and inferior temporal convexity. Most "motion" neurons showed strong response modulation during the course of an action, thus responding to action kinematics. "Motion" neurons displayed a greater average selectivity for these simple arm actions than did "snapshot" neurons. We suggest that the "motion" neurons code for visual kinematics, whereas the "snapshot" neurons code for form/posture, and that both can contribute to action recognition, in agreement with computation models of action recognition.

Actions can be understood based on form cues (e.g., static body posture) as well as motion cues (e.g., gait patterns). A fundamental debate centers on the question of whether the functional and neural mechanisms processing these two types of cues are dissociable. Here, using fMRI, psychophysics, and transcranial magnetic stimulation (TMS), all within the same human participants, we show that mechanisms underlying body form and body motion processing are functionally and neurally distinct. Multivoxel fMRI activity patterns in the extrastriate body area (EBA), but not in the posterior superior temporal sulcus (pSTS), carried cue invariant information about the body form of an acting human. Conversely, multivoxel patterns in pSTS, but not in EBA, carried information about the body motion of the same actor. In a psychophysical experiment, we selectively impaired body form and body motion discriminations by manipulating different visual cues: misaligning the ellipses that made up a dynamic walker stimulus selectively disrupted body form discriminations, while varying the presentation duration of the walker selectively affected body motion discriminations. Finally, a TMS experiment revealed causal evidence for a double-dissociation between neural mechanisms underlying body form and body motion discriminations: TMS over EBA selectively disrupted body form discrimination, whereas TMS over pSTS selectively disrupted body motion discrimination. Together, these findings reveal complementing but dissociable functions of EBA and pSTS during action perception. They provide constraints for theoretical and computational models of action perception by showing that action perception involves at least two parallel pathways that separately contribute to the understanding of others' behavior.

Temporal cortical neurons are known to respond to visual dynamic-action displays. Many human psychophysical and functional imaging studies examining biological motion perception have used treadmill walking, in contrast to previous macaque single-cell studies. We assessed thecodingoflocomotioninrhesusmonkey(Macacamulatta)temporalcortexusingmoviesofstationarywalkers,varyingbothformandmotion (i.e.,differentfacingdirections)orvaryingonlytheframesequence(i.e.,forwardvsbackwardwalking).Themajorityofsuperiortemporalsulcus and inferior temporal neurons were selective for facing direction, whereas a minority distinguished forward from backward walking. Support vector machines using the temporal cortical population responses as input classified facing direction well, but forward and backward walking less so. Classification performance for the latter improved markedly when the within-action response modulation was considered, reflecting differences in momentary body poses within the locomotion sequences. Responses to static pose presentations predicted the responses during the course of the action. Analyses of the responses to walking sequences wherein the start frame was varied across trials showed that some neurons also carried a snapshot sequence signal. Such sequence information was present in neurons that responded to static snapshot presentationsandinneuronsthatrequiredmotion.Ourdatasuggestthatactionsareanalyzedbytemporalcorticalneuronsusingdistinctmechanisms. Most neurons predominantly signal momentary pose. In addition, temporal cortical neurons, including those responding to static pose, are sensitive to pose sequence, which can contribute to the signaling of learned action sequences.