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At least 1,369 records · Page 76Linked to original sources
PubMed1983

On the failure of spatiotemporal interpolation: a filtering model.

A vernier target moving discontinuously between discrete spatial stations is seen as having a spatial offset if its component bars are presented at spatially aligning stations with a temporal delay (the interpolation effect). In agreement with previous work, we find that the threshold for detecting this virtual spatial offset is the same as that for detecting an explicit spatial offset, provided that certain spatial and temporal constraints are met. Interpolation is a completely efficient process provided that the spatial interval between stations in the movement trajectory is less than 3-4 min arc, but thereafter it shows a gradual decline in efficiency. This paper presents a model of the decline in efficiency which arises, we suggest, not because of a failure of sampling to represent the original stimulus, but because of a progressive mismatch between the sampled signal and the bandwidth of the spatial filters involved in interpolation. Our model is compared with the constant velocity model of Fahle & Poggio [Proc. R. Soc. Lond. B 213, 451-477 (1981)].

Humans↗
PubMed1989

Receptive field properties of human motion detector units inferred from spatial frequency masking.

This study was designed to investigate the spatial frequency selectivity and spatial structure of receptive fields of motion sensitive mechanisms in human vision. Spatial frequency selectivity was inferred from masking measurements, using dynamic test and mask stimuli. For test frequencies between 0.025 and 15.0 c/deg, maximal masking occurred when the mask frequency matched that of the test, suggesting that the test was detected by mechanisms tuned to (or near to) that frequency. For tests below 0.025 c/deg or above 15.0 c/deg, maximal masking occurred at 0.025 and 15.0 c/deg, respectively, suggesting that there exist no mechanisms selective to frequencies outside these limits. A masking model, suitable for interpreting results obtained with drifting test stimuli, was developed and used to calculate spatial frequency selectivity functions from masking data. Assuming small signal linearity, and a constant phase spectrum, the selectivity functions were inverse-Fourier transformed to yield estimates of the extent and structure of receptive fields. Field width was found to vary with test spatial frequency from 5.8 deg at 0.03 c/deg to 0.05 deg at 10.0 c/deg. These estimates were compared with width estimates previously obtained by a summation technique (Anderson & Burr, 1987), and found to be similar over a wide range of spatial frequencies (2.5 log units). Gabor functions provided a reasonable fit to the calculated field profiles at high spatial frequencies (above 1.0 c/deg), but not at low frequencies.

Fourier Analysis↗
PubMed1991

The effect of spatial parameters on oscillatory movement displacement thresholds.

Earlier work has established that oscillatory movement displacement thresholds (OMDT) are a form of hyperacuity. There is speculation that the mechanism determining OMDT, like motion perception in general, involves direct motion sensing at high temporal frequencies of oscillation and spatial localization processes (from which motion is inferred) at low temporal frequencies, which are both hyperacuities in their own right. OMDT were determined, for three experienced observers, over the temporal frequency range 1-15 Hz, for three stimulus lengths and three stimulus widths. Both decreasing stimulus length and decreasing stimulus width increased OMDT at all temporal frequencies. Furthermore, the resulting functions consistently exhibit a "kink" in the temporal frequency midrange. The results are interpreted as evidence that there are two subsystems involved in the analysis of visual motion with the kink indicating the transition where one system begins to predominate over the other.

Humans↗
PubMed1992

Spatiotemporal factors in infant position sensitivity: single bar stimuli.

The ability of 3-month-old human infants to detect the presence of spatial misalignments in single-bar stimuli was investigated in a series of spatiotemporal stimulus manipulations. Both discrete and sinusoidal positional offsets, either stationary or temporally modulated, were presented using the forced-choice preferential looking technique. When discrete offsets were presented in alteration with the absence of offsets, thresholds were a factor of two worse than previous estimates of infant vernier acuity using grating stimuli. However, when offsets (discrete or sinusoidal) were presented in continuous motion, mean threshold was 22-24 arcmin, comparable to previous estimates using gratings with moving discrete offsets. For stimuli containing continuous motion, lowering velocity reduced infants' positional sensitivity. However, when either velocity or temporal frequency was held constant, the most important determinant of positional sensitivity was the sharpness of the offsets. The results from the temporally-modulated stimuli suggest that a simple local flicker mechanism cannot account for sensitivity when continuous motion is used in the stimulus; rather, a local motion mechanism may govern sensitivity in these conditions. We characterize this mechanism as one of "local motion" because it does have a position-sensitive component. The two stationary stimulus conditions in the present study indicate that infants can use a position-sensitive mechanism when no temporal modulation is present.

Humans↗
PubMed1993

The lateral inhibition of perceived contrast is indifferent to on-center/off-center segregation, but specific to orientation.

UNLABELLED: When a central test patch C, composed of an isotropic spatial texture, is surrounded by a texture field S, the perceived contrast of C depends substantially on the contrast of the surround S. When C is surrounded by a high contrast texture with a similar spatial frequency content, it appears to have less contrast than when it is surrounded by a uniform field. Here, we employ two novel textures: T+ which is designed to selectively stimulate only the on-center system, and T-, the off-center system. When C and S are of type T+ and T-, the reduction of C's apparent contrast does not vary with the combination of T+, T-. This demonstrates that the reduction of C's apparent contrast is mediated by a mechanism whose neural locus is central to the interaction between on-center and off-center visual systems. We further demonstrate orientation specificity: the reduction of grating C's apparent contrast by a surround grating S, of the same spatial frequency is greatest when C and S have equal orientation. Using dynamically phase-shifting sinusoidal gratings of 3.3, 10 and 20 c/deg, we measured reduction of apparent contrast using different contrast-combinations of C and S. RESULTS: (1) S gratings, both parallel and perpendicular to C, cause a reduction in C's apparent contrast relative to a uniform surround. (2) In all of the viewing conditions, the reduction of apparent contrast induced by the parallel surrounds was at least as great as that induced by the perpendicular surrounds. Often it was much greater. (3) Orientation specificity increases with increasing spatial frequency and with decreasing stimulus contrast.

Contrast Sensitivity↗