1. We have analyzed receptive fields (RFs) of directionally selective (DS) complex cells in the striate cortex of the cat. We determined the extent to which the DS of a complex cell depends on spatially identifiable subunits within the RF by studying responses to an optimally oriented, three-luminance-valued, gratinglike stimulus that was spatiotemporally randomized. 2. We identified subunits by testing for nonlinear spatial RF interactions. To do this, we calculated Wiener-like kernels in a spatial superposition test that depended on two RF positions at a time. The spatial and temporal separation of light and dark bars at these two positions varied over a spatial range of 8 degrees and a temporal range of +/- 112 ms in increments of 0.5 degree and 16 ms, respectively. 3. DS responses in complex cells cannot be explained by their responses to single light or dark bars because any linear superposition of responses whose time course is uniform across space shows no directional preference. 4. Nonlinear interactions between a flashed reference bar that is fixed in position and a second bar that is flashed at surrounding positions help explain DS by showing multiplicative-type facilitation for bar pairs that mimic motion in the preferred direction and suppression for bar pairs that mimic motion in the null direction. Interactions in the preferred direction have an optimal space/time ratio (velocity), exhibited by elongated, obliquely oriented positive domains in a space-time coordinate frame. This relationship is inseparable in space-time. The slope of the long axis specifies the preferred speed, and its negative agrees with the most strongly suppressed speed in the opposite direction. 5. When the reference bar position is moved across the RF, the spatiotemporal interaction moves with it. This suggests the existence of a family of nearly uniform subunits distributed across the RF. We call the subunit interaction, as averaged across the RF, the "motion kernel" because its spatial and temporal variables are those necessary to specify the velocity, the only parameter that distinguishes a moving image from a temporally modulated stationary image. The nonlinear interaction shows a spatial periodicity, which suggests a mechanism of velocity selectivity for moving extended images.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging and Alzheimer's disease (AD) are associated with declines in the visual perception of self-movement that undermine navigation and independent living. We studied 214 subjects' heading direction and speed discrimination using the radial patterns of visual motion in optic flow. Young (YA), middle-aged (MA), and older normal (ON) subjects, and AD patients viewed optic flow in which we manipulated the motion coherence, spatial texture, and temporal periodicity composition of the visual display. Aging and AD were associated with poorer heading and speed perception at lower temporal periodicity, with smaller effects of spatial texture. AD patients were particularly impaired by motion incoherence created by adding randomly moving dots to the optic flow. We conclude that visual motion processing is impaired by distinct mechanisms in aging and the transition to AD, implying distinct neural mechanisms of impairment.

Temporal acuity for brief gaps in noise was studied in mice of different ages (1-36 months) from strains with differing susceptibility to age-related hearing loss, using reflex modification audiometry. Prepulse inhibition of the acoustic startle reflex (ASR) increased with gap depth (GD: 10-40 dB in 70 dB SPL noise) and lead time (LT: 1-15 ms). The increase in inhibition with LT followed an exponential function in which the two parameters, asymptotic inhibition (AINH) and the time constant (tau), were both affected by GD. AINH rapidly declined from 1 to 6 and then to 18 months of age in C57BL/6J mice with progressively severe hearing loss, but first increased with maturation and then gradually declined beyond 6-12 months of age in CBA/CaJ and CBA x C57BL Fl-hybrid mice, which show no apparent change in sensory function at these ages. In contrast, tau was unaffected by hearing loss or by age, this suggesting that age-related changes in this form of temporal acuity occur because of a reduction in the efficiency with which gaps are centrally processed, not from any reduced ability to follow their rapid shift in noise level.

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Navigation relies on the neural processing of sensory cues about observer self-movement and spatial location. Neurons in macaque dorsal medial superior temporal cortex (MSTd) respond to visual and vestibular self-movement cues, potentially contributing to navigation and orientation. We moved monkeys on circular paths around a room while recording the activity of MSTd neurons. MSTd neurons show a variety of sensitivities to the monkey's heading direction, circular path through the room, and place in the room. Changing visual cues alters the relative prevalence of those response properties. Disrupting the continuity of self-movement paths through the environment disrupts path selectivity in a manner linked to the time course of single neuron responses. We hypothesize that sensory cues interact with the spatial and temporal integrative properties of MSTd neurons to derive path selectivity for navigational path integration supporting spatial orientation.