Jack Broerse

Spatial determinants of the aftereffect of seen motion

Observers were required to track the perceived motion of a stationary grating viewed following exposure to a moving grating. The size of the motion aftereffect was complexly controlled by relationships between the velocity and periodicity of the adaptation grating and the periodicity of the test grating. In addition the motion aftereffect was selective to the direction of motion of the adaptation grating. These results are considered in terms of a model attributing the motion aftereffect to selective postexcitatory suppression in the response of directionally-excited motion detectors in the visual system.

The influence of imagery ability on color aftereffects produced by physically present and imagined induction stimuli

Two methods of induction were used to produce orientation-contingent color aftereffects for observers assigned to one of three groups (high, medium, and low) on the basis of self-rated imagery ability. In Experiment 1, observers were required to make magnitude estimates of color aftereffects following inspection of stimulus patterns normally used to produce McCollough effects (e.g., red vertical contours, green horizontal contours). Experiment 2 was a partial replication of Experiment 1, with additional induction conditions in which observers were required to imagine the presence of appropriately oriented contours when particular homogeneous color patches were presented. The results indicated that self-rated imagery ability was not a significant factor in differentiating between observers’ performance when orthodox induction procedures were used (Experiment 1). In addition, there were no reliable indications (Experiment 2) that imagined stimulus attributes can be effectively substituted for real stimulus attributes in order to produce orientation-contingent color aftereffects. The results are discussed in terms of their implications for the use of imagery-induced perceptual phenomena as a paradigm for investigating the possibility of common neural mechanisms in perception and imagination; in addition, the general implications of the results for understanding the functional significance of self-reported imagery ability are examined.

Looking at the task in hand: vergence eye movements and perceived size

Abstract A retinal afterimage of the hand changes size when the same unseen hand is moved backwards and forwards in darkness. We demonstrate that arm movements per se are not sufficient to cause a size change and that vergence eye movements are a necessary and sufficient condition for the presence of the illusory size change. We review previous literature to illustrate that changing limb position in the dark alters vergence angle and we explain the illusion via this mechanism. A discussion is provided on why altering limb position causes a change in vergence and we speculate on the underlying mechanisms.

Investigations of perception and imagery using CAEs: The role of experimental design and psychophysical method

Orientation-contingent color aftereffects (CAEs) were measured using 174 observers ran-domly allocated to one of eight experimental conditions formed by the factorial combination of three two-level factors. These were experimental design (pretest and posttest measures taken vs. posttest measures taken alone), psychophysical method (forced-choice vs. magnitude estimation), and inspection procedure (inspection of real contours vs. inspection of imagined contours). In addition to color responses, written introspective reports of the strategies used in providing color responses were obtained. As a basis for the rejection of observers using inappropriate strategies, reported strategies were coded as appropriate or inappropriate. An analysis of the color reports of all observers was performed, as well as two analyses of the color reports of only those observers identified as using appropriate response strategies. In all cases, the results showed that CAEs were reported after inspection of real contours irrespective of the experimental design or psychophysical method used. There were no significant CAEs reported in any conditions that involved the imagination of contours. A separate analysis of the relationship between observers’ response strategies, the coding of these strategies, and observers’ actual color reports revealed some problems concerning the use of such introspective techniques. The implications of these findings are discussed in terms of theoretical assumptions about the nature of imagery and perception.

Emmert's Law in the Ames Room

The Ames distorted room illusion, in which the perceived sizes of objects placed within the room differ from their objective sizes, has been used to support arguments for indirect perception. A study is reported in which Emmerts law of the apparent size of after-images was examined in relation to the Ames rooms illusory alteration of apparent and actual distances. Size judgments of afterimages projected into the Ames room were compared with control conditions in which both actual and apparent afterimage projection distances were reproduced. Results indicate that Emmerts law may not provide a simple geometrical relationship between proximal image size and actual viewing distance, and that the processes involved in making afterimage size judgments are similar to those processes involved in making size judgments of ‘real world’ objects.

Monocular rivalry occurs without eye movements

Georgeson and Phillips (1980) propose that monocular rivalry (MR; perceptual alternations between two optically superimposed gratings of different orientations) occurs because “eye movements cause partial cancellations between the stimulus pattern and the negative after-image from a preceding fixation” (p. 1010). They conclude that if their proposition is correct, then “MR reveals very little about the mechanisms of pattern vision” (p. 1012). The evidence adduced by Georgeson and Phillips (1980) in support of their claim consists of a computer simulation of the well known angular function of MR (small angular differences, low alternation rates between superimposed gratings; large angular differences, high ahernation rates) which Georgeson and Phillips (1980) replicate with real observers. The computer simulation is based on the assumption that MR is dependent on the ratio of the effective contrast of the two gratings comprising the stimulus pattern (where effective contrast is defined as the contrast of an original grating modulated by the contrast of the negative after-image of this grating produced following an eye movement). When the ratio of effective contrast is greater than P, or less than l/P (where P is some arbitary criterion value) the computer “sees” one or other of the two gratings as dominant. By judicial selection of P, Georgeson and Phillips (1980) are able to produce data which mimic the MR curves of human observers. On this basis, they conclude that change in effective contrast due to eye movements is the mechanism of MR. In this note we report data which indicate that MR can occur in the absence of eye movements, and are thus inconsistent with Georgeson and Phillips (1980) proposition. The most direct way of investigating the role of eye movements in MR is to monitor and/or controt eye movements during MR. We are not equipped to do this (neither were Georgeson and Phillips, 1980) and decided instead to investigate the angular function of MR while eliminating eye movements (hence their possible role in MR as suggested by Georgeson and Phillips, 1980). The method (using after-images as stabilized retinal stimuli for MR) was suggested by experiments reported by Atkinson (1973) who found that the synchrony of visibility of adjacent afterimages was dependent on their relative orientation. In addition, Wade (1978) reviews a deal of evidence suggesting that after-images and “real” contours may be processed by the same mechanisms. For these reasons we felt the use of after-images in the study of MR was warranted. Monocular negative after-images were induced in five 5s who had normal or corrected vision and who were naive about the purpose of the experiment. Subjects were tested in three sessions (of 5-10 min) separated by at least 120min. In each session successive after-images of a vertical (0’) and a non-vertical (15, 45’ or 75”) square wave grating (4c/deg) were generated by two successive discharges (separated by about 4sec) of a Sylvania Blue Dot Flashcube (manufacturer’s specification for total light output was approx. 2 x lo3 Beam Candle Power Seconds, and for flash duration was approximately 35 msec). The stimulus grating had been photographically reproduced on a transparent medium and housed in a circular frame which could be manually rotated to any desired orientation by E. The frame was mounted at eye level behind a matte black board containing a circular aperture (subtending 4 deg) through which the grating was visible, and the entire display was curtained to provide homogeneous visual surroundings. Subjects were seated 65 cm from the display and an adjustable chin rest was used in conjunction with a central fixation point to maintain constant head and eye position. After Ss left eye had been occluded, E adjusted the orientation of the grating for the first stimulus presentation and reset the orientation during the insterstimulus interval. Following the presentation of the second grating, Ss were required to look to the homogeneous visual surround and record the visibility of both after-image orientation components as well as any composite after-images in which both gratings were equally visible. Responses were recorded by depressing one of three buttons connected to a Cromemko (Model Z-2D) microprocessor: when both orientations were equally visible, the middle button was depressed and held down; if one orientation was more visible than the other, the appropriate side buttons were depressed. Presentation orders for orientations within pairs were randomized, as were the orders of pairs across sessions. A MR alternation was defined as any change in reported perceptuat dominance involving the three possible percepts (i.e. vertical, non-verticai, compo-. site). Figure 1 shows the mean duration of afterimages for the three angular separations, together

Developmental changes in the incidence and likelihood of simultaneous talk during the first two years: a question of function

Global tendencies for the relative absence of covocalization (simultaneous talk) have been identified in both conversations between adult partners and conversations between mothers and their infants; in each case, the alternating mode in which one partner speaks at a time is predominant. The present investigation examined the timing of the partners talk in mother-infant engagements over infant age to determine whether: (a) variations occur in the incidence of the alternating mode; and (b) variations occur in the extent to which the alternating mode predominates. Conversations involving a total of 48 mothers and their infants aged from 0;3 to 2;0 were investigated at each of eight infant ages (0;3, 0;6, 0;9, 1;0, 1;3, 1;6, 1;9 and 2;0). The results indicated that, within a global tendency for the relative absence of covocalization, there was: (a) a curvilinear tendency for the incidence of covocalization to decrease over the first 18 months, and then to increase; and (b) a linear tendency for the extent to which the alternating mode predominates to increase over age. These changes are interpreted as reflecting the facilitative effects of covocalization in the case of young preverbal infants, and the need for the alternating, turn-taking pattern to predominate as mutual comprehension becomes possible in conversations between mothers and their older infants.

Making ambiguous displays unambiguous: The influence of real colors and colored aftereffects on perceptual alternation

The relationship between orientation-contingent colored aftereffects (CAEs) and perceptual alternation of ambiguous displays was investigated in three experiments. In all experiments, the ambiguous test display consisted of vertical and horizontal contours that either could be perceived as separate surfaces (the diamond organization) or could be combined to form upright and inverted Us (the rectangles organization). In Experiments 1 and 2, observers inspected the test display when it was achromatic, when it was colored in a manner consistent with the appearance of CAEs, and when it was colored in a non-CAE manner. In Experiment 3, the test display was inspected monocularly before and after monocular color-orientation adaptation. The achromatic viewing conditions of Experiments 1 and 2, both monocular preadaptation achromatic viewing conditions of Experiment 3, and the postadaptation achromatic viewing condition involving the nonadaptation eye in Experiment 3 produced essentially the same results: reports of approximately equal duration for the diamond and rectangles organizations and steady rates ofperceptual alternation. The non-CAE color conditions of Experiment 2 produced a similar pattern of results. However, the color condition of Experiment 1 (i.e., simulated CAEs), and the postadaptation condition involving the adaptation eye in Experiment 3 produced a decrement in the perceptual alternation rate, with the diamond organization being reported almost exclusively. These results indicate that ambiguous displays can be made unambiguous by altering the appearance of the displays either through the addition of appropriate colors or by exposing observers to appropriate adaptation conditions. These results are consistent with stimulus-bound explanations of both CAEs and perceptual alternation.

Colored aftereffects contingent upon global transformations

Dodwell and Humphrey (Psychol. Rev. 97, 78-79, 1990) have argued that colored aftereffects (CAEs) are contingent on the global geometries of local orientation and edge components in induction patterns. The spatiotopic dimensions underlying these geometries are derived from previous applications of the mathematics of continuous transformations (Lie groups) to the stimulus dynamics of perceptual invariance. In these terms, complementary CAEs induced by inspecting alternating colored bullseyes and spokes are attributed to the adaptation of orthogonal spatiotopic channels; the former characterized by vectorfields dealing with object invariance under rotation, and the latter by vectorfields dealing with object invariance under dilation. Importantly, the pattern-contingent CAEs induced in this pair of pattern channels are independent of CAEs induced in channels characterized by vectorfields dealing with object invariance under vertical and horizontal translations (e.g. vertical and horizontal gratings). The present investigation examined whether CAEs contingent on the stimulus dynamics originally used to define these spatiotopic channels exhibit comparable properties. It is confirmed that pairs of complementary CAEs can be induced simultaneously with radially expanding and contracting bullseyes, and clockwise- and counter-clockwise-rotating spokes (Experiment 1); and it is shown that these CAEs combine linearly in generalizing to and from the radial and rotational components of spiral motion (Experiments 2 and 3 respectively). In direct contrast to findings for CAEs induced with stationary versions of these patterns, however, it is demonstrated that complementary pairs of CAEs induced by expanding and contracting bullseyes interact with CAEs induced by left- and right-moving vertical gratings, suggesting that the components of motion underlying these contingent effects are locally and not globally determined (Experiment 4).

Orientation-specific aftereffects and illusions in the perception of brightness*

Orientation-specific brightness aftereffects were found when vertical and horizontal gratings of the same space-average luminance were viewed following alternate exposure to vertical and horizontal gratings that differed in space-average luminance. The vertical test grating appeared bright following exposure to a dim vertical grating, and dim after a bright vertical grating had been viewed. This aftereffect did not occur when the adaptation gratings had been seen by one eye and the test gratings by the other eye. An orientation-specific illusion in the perception of brightness was also found, with the white sectors of a vertical grating appearing brighter against a background of horizontal lines than they did against a background of vertical lines. Both distortions imply that there are detectors in the human visual system that are conjointly tuned to luminance and contour orientation.