John M. Foley

Contrast masking in human vision

Contrast masking was studied psychophysically. A two-alternative forced-choice procedure was used to measure contrast thresholds for 2.0 cpd sine-wave gratings in the presence of masking sine-wave gratings. Thresholds were measured for 11 masker contrasts spanning three log units, and seven masker frequencies ranging +/- one octave from the signal frequency. Corresponding measurements were made for gratings with horizontal widths of 0.75 degrees (narrow fields) and 6.0 degrees (wide fields). For high contrast maskers at all frequencies, signal thresholds were related to masking contrast by power functions with exponents near 0.6. For a range of low masking contrasts, signal thresholds were reduced by the masker. For the wide fields, high contrast masking tuning functions peaked at the signal frequency, were slightly asymmetric, and had approximately invariant half-maximum frequencies that lie 3/4 octave below and 1 octave above the signal frequency. The corresponding low contrast tuning functions exhibited peak threshold reduction at the signal frequency, with half-minimum frequencies at roughly +/- 0.25 octaves. For the narrow fields, the masking tuning functions were much broader at both low and high masking contrasts. A masking model is presented that encompasses contrast detection, discrimination, and masking phenomena. Central constructs of the model include a linear spatial frequency filter, a nonlinear transducer, and a process of spatial pooling that acts at low contrasts only.

Contrast detection and near-threshold discrimination in human vision

Abstract Forced-choice psychometric functions were determined for the detection of sinewave gratings and contrast discrimination of near-threshold gratings at spatial frequencies of 0.5, 2 and 8 c/deg. Detection psychometric functions all had the same S-shaped form. Discrimination functions were almost linear except at the upper end. Both sets of data can be described well by a detection model with a positively accelerating relation between contrast and mean decision variable and a differencing decision rule. Results of a paired comparisons experiment were consistent with the model and indicate that decision variable variance is nearly constant over the range of contrasts used in these experiments. The implications of these results for several models of contrast detection and discrimination are considered.

Visually directed pointing as a function of target distance, direction, and available cues*

In pointing at visual targets without sight of the hand, large errors occur. There is a tendency to overreach targets, and this tendency is much greater (about 25 em) when convergence is the only cue to distance than when there are many cues (2–11 cm). Angular errors of up to 10 deg also occur. These tend to be to the side opposite the sighting eye, when the favored hand is used. The variance of the pointing response with convergence alone is reduced by approximately half with the introduction of several spatial cues. These results are interpreted as indicating that, for a target within the reach of the arm and with convergence alone as a cue, the depth signal produced by the visual system corresponds to a greater distance than that produced when many cues are available. The results are also consistent with the hypothesis that perceived direction tends to approximate direction from the sighting eye.

Visual perception of extent and the geometry of visual space.

The question of how perceived extents are related to the corresponding physical extents is a very old question that has not been satisfactorily answered. The common model is that perceived extent is proportional to the product of image size and perceived distance. We describe an experiment that shows that perceived extents are substantially larger than this model predicts. We propose a model that accounts for our results and a large set of other results. The principal assumption of the model is that, in the computation of perceived extent, the visual angle signal undergoes a magnifying transform. Extent is often perceived more accurately than the common model predicts, so the computation is adaptive. The model implies that, although the perception of location and the perception of extent are related, they not related by Euclidean geometry, nor by any metric geometry. Nevertheless, it is possible to describe the perception of location and extent using a simple model.

Effect of Distance Information and Range on Two Indices of Visually Perceived Distance

Subjects made verbal reports of the perceived distance of near visual targets and indicated their distances by pointing at them with an unseen hand. The targets were presented under three stimulus conditions: monocular, binocular, and multicue. Two ranges of target distance were studied in separate experiments: 11–39 cm and 21–33 cm. When reciprocals of both response distance and target distance were plotted, all functions were approximately linear. The verbal and manual responses (distance indices) differed greatly, but were found to be related by a constant transform. The range of target distances had no effect on either index. The standard deviation of the manual response was about half that of the verbal response. Stimulus condition had a large effect, with the multicue condition producing the greatest change in indicated distance, and the monocular condition the least. The results are discussed with reference to the definition of perceived distance, and hypotheses concerning the integration of distance information.

Spatial attention: effect of position uncertainty and number of distractor patterns on the threshold-versus-contrast function for contrast discrimination

Our goal is to integrate knowledge about contrast discrimination with knowledge about spatial attention effects. An experiment is described that measures the effects of position uncertainty, number of distractors, and contrast on the contrast discrimination threshold in a fully crossed factorial design. The threshold-versus-contrast function is nonmonotonic in all conditions, decreasing and then increasing as contrast increases. Increasing uncertainty and/or the number of distractors increases thresholds, and there are interactions among the three variables indicating that uncertainty and distractor number have different effects on detection as distractor contrast varies. The results are well accounted for by a model that combines a nonlinear excitation/divisive inhibition model of pattern mechanisms with a noise-limited model of the decision process.

The size-distance relation and intrinsic geometry of visual space: Implications for processing

Abstract When a perceived frontal size is matched to a perceived egocentric distance with only primary cues available, the corresponding physical size-distance ratio is about 0.5. Perceived visual angle exceeds physical angle by much less. It follows that under these conditions the visual space is not Euclidean. This phenomenon is demonstrated in two experiments. It may be the basic phenomenon responsible for the non-Euclidean character of visual space. It is, however, inconsistent with the assumption of homogeneity of the Luneburg-Blank theory. When additional cues to distance are introduced, the physical ratio is set much closer to the perceived ratio with no corresponding change in the perceived magnitude of visual angles. The results are interpreted as disconfirming the hypothesis that perceived size is inferred from visual angle and perceived distance. Rather they suggest two independent modes of spatial information processing.

New model of human luminance pattern vision mechanisms: analysis of the effects of pattern orientation, spatial phase, and temporal frequency

Models of human pattern vision mechanisms are examined in light of new results in psychophysics and single-cell recording. Four experiments on simultaneous masking of Gabor patterns by sinewave gratings are described. In these experiments target contrast thresholds are measured as functions of masker contrast, orientation, spatial phase, and temporal frequency. The results are used to test the theory of simultaneous masking proposed by Legge and Foley that is based on mechanisms that sum excitation linearly over a receptive field and produce a response that is an s-shaped transform of this sum. The theory is shown to be inadequate. Recent single-cell-recording results from simple cells in the cat show that these cells receive a broadband divisive input as well as an input that is summed linearly over their receptive fields. A new theory of simultaneous masking based on mechanisms with similar properties is shown to describe the psychophysical results well. Target threshold vs masker contrast (TvC) functions for a set of target-masker pairs are used to estimate the parameters of the theory including the excitatory and inhibitory sensitivities of the mechanisms along the various pattern dimensions. The human luminance pattern vision mechanisms, unlike most of the cells, do not saturate at high contrast.

Analysis of the Effect of Pattern Adaptation on Pattern Pedestal Effects: A Two-process Model

Pattern contrast thresholds for vertical Gabor patterns were measured on pattern pedestals that were vertical or horizontal. Contrast of the pedestal was varied to measure the function relating target contrast threshold to pedestal contrast (TvC function). TvC functions were measured without an adaptor and after adaptation to vertical, horizontal and plaid patterns. For a pedestal with the same orientation as the target, the vertical and plaid adapters increased thresholds at low pedestal contrasts, but not high. For the pedestal orthogonal to the target, the same two adaptors increased thresholds over the whole range of pedestal contrasts. These asymmetric effects are described by a model of adaptation and masking derived from a model of masking (Foley, 1994a) by allowing two parameters to vary with the adapt state; one of them is an additive parameter in the denominator of the response function, which can be interpreted as adaptor-produced divisive inhibition that persists after adaptor offset; the other is the sensitivity to pedestal-produced divisive inhibition, which is changed by adaptation for the pedestal orthogonal to the target. Other models do not account for both effects.

Effects of voluntary eye movement and convergence on the binocular appreciation of depth.

Scaling techniques were employed to establish the relation between perceived distance ratio and physical distance ratio. Measurements were made both with and without free eye movement and under two states of convergence. The results were confirmed using a matching technique. With free eye movement, the perceived ratio is a monotonic increasing function of the physical ratio. Without eye movement, the perceived ratio generally increases, then decreases, as the physical ratio increases. For a given physical ratio, perceived distance ratio is less in the absence of voluntary eye movements. Convergence produces depth micropsia when eye movements are permitted, but not in their absence.