Experimental test of spatial updating models for monkey eye-head gaze shifts

Spectral-temporal processing of naturalistic sounds in monkeys and humans

Human speech and vocalizations in animals are rich in joint spectrotemporal (S-T) modulations, wherein acoustic changes in both frequency and time are functionally related. In principle, the primate auditory system could process these complex dynamic sounds based on either an inseparable representation of S-T features or, alternatively, a separable representation. The separability hypothesis implies an independent processing of spectral and temporal modulations. We collected comparative data on the S-T hearing sensitivity in humans and macaque monkeys to a wide range of broadband dynamic spectrotemporal ripple stimuli employing a yes-no signal-detection task. Ripples were systematically varied, as a function of density (spectral modulation frequency), velocity (temporal modulation frequency), or modulation depth, to cover a listener's full S-T modulation sensitivity, derived from a total of 87 psychometric ripple detection curves. Audiograms were measured to control for normal hearing. Determined were hearing thresholds, reaction time distributions, and S-T modulation transfer functions (MTFs), both at the ripple detection thresholds and at suprathreshold modulation depths. Our psychophysically derived MTFs are consistent with the hypothesis that both monkeys and humans employ analogous perceptual strategies: S-T acoustic information is primarily processed separable. Singular value decomposition (SVD), however, revealed a small, but consistent, inseparable spectral-temporal interaction. Finally, SVD analysis of the known visual spatiotemporal contrast sensitivity function (CSF) highlights that human vision is space-time inseparable to a much larger extent than is the case for S-T sensitivity in hearing. Thus, the specificity with which the primate brain encodes natural sounds appears to be less strict than is required to adequately deal with natural images.NEW & NOTEWORTHY We provide comparative data on primate audition of naturalistic sounds comprising hearing thresholds, reaction time distributions, and spectral-temporal modulation transfer functions. Our psychophysical experiments demonstrate that auditory information is primarily processed in a spectral-temporal-independent manner by both monkeys and humans. Singular value decomposition of known visual spatiotemporal contrast sensitivity, in comparison to our auditory spectral-temporal sensitivity, revealed a striking contrast in how the brain encodes natural sounds as opposed to natural images, as vision appears to be space-time inseparable.

Modeling auditory-visual evoked eye-head gaze shifts in dynamic multisteps

In dynamic visual or auditory gaze double-steps, a brief target flash or sound burst is presented in midflight of an ongoing eye-head gaze shift. Behavioral experiments in humans and monkeys have indicated that the subsequent eye and head movements to the target are goal-directed, regardless of stimulus timing, first gaze shift characteristics, and initial conditions. This remarkable behavior requires that the gaze-control system 1) has continuous access to accurate signals about eye-in-head position and ongoing eye-head movements, 2) that it accounts for different internal signal delays, and 3) that it is able to update the retinal ( TE) and head-centric ( TH) target coordinates into appropriate eye-centered and head-centered motor commands on millisecond time scales. As predictive, feedforward remapping of targets cannot account for this behavior, we propose that targets are transformed and stored into a stable reference frame as soon as their sensory information becomes available. We present a computational model, in which recruited cells in the midbrain superior colliculus drive eyes and head to the stored target location through a common dynamic oculocentric gaze-velocity command, which is continuously updated from the stable goal and transformed into appropriate oculocentric and craniocentric motor commands. We describe two equivalent, yet conceptually different, implementations that both account for the complex, but accurate, kinematic behaviors and trajectories of eye-head gaze shifts under a variety of challenging multisensory conditions, such as in dynamic visual-auditory multisteps.

Transsaccadic processing: stability, integration, and the potential role of remapping

While our frequent saccades allow us to sample the complex visual environment in a highly efficient manner, they also raise certain challenges for interpreting and acting upon visual input. In the present, selective review, we discuss key findings from the domains of cognitive psychology, visual perception, and neuroscience concerning two such challenges: (1) maintaining the phenomenal experience of visual stability despite our rapidly shifting gaze, and (2) integrating visual information across discrete fixations. In the first two sections of the article, we focus primarily on behavioral findings. Next, we examine the possibility that a neural phenomenon known as predictive remapping may provide an explanation for aspects of transsaccadic processing. In this section of the article, we delineate and critically evaluate multiple proposals about the potential role of predictive remapping in light of both theoretical principles and empirical findings.

Brain circuits underlying visual stability across eye movements-converging evidence for a neuro-computational model of area LIP

The understanding of the subjective experience of a visually stable world despite the occurrence of an observer's eye movements has been the focus of extensive research for over 20 years. These studies have revealed fundamental mechanisms such as anticipatory receptive field (RF) shifts and the saccadic suppression of stimulus displacements, yet there currently exists no single explanatory framework for these observations. We show that a previously presented neuro-computational model of peri-saccadic mislocalization accounts for the phenomenon of predictive remapping and for the observation of saccadic suppression of displacement (SSD). This converging evidence allows us to identify the potential ingredients of perceptual stability that generalize beyond different data sets in a formal physiology-based model. In particular we propose that predictive remapping stabilizes the visual world across saccades by introducing a feedback loop and, as an emergent result, small displacements of stimuli are not noticed by the visual system. The model provides a link from neural dynamics, to neural mechanism and finally to behavior, and thus offers a testable comprehensive framework of visual stability.