Ethel Matin

Saccadic overhead : information-processing time with and without saccades

Information-processing time was compared for serial and spatially distributed visual presentations with performance measures that permit the separation of total time into its during-display and post-display components. For all subjects, there was a significant saccadic overhead, that is, less time was required with the serial format, which allowed data access without eye movements. However, the magnitude of the overhead decreased as task complexity increased. All subjects were able to exercise some control over the distribution of total processing time, trading off short during-display times with longer post-display times and vice versa.

Visual perception of direction when voluntary saccades occur. I. Relation of visual direction of a fixation target extinguished before a saccade to a flash presented during the saccade

In experiments designed to clarify the mechanisms underlying the normal stability of visual direction for stationary objects when voluntary saccades occur, Ss reported on the horizontal visual direction of a brief test [lash presented when the eye was at a specific point in the saccade (the trigger point) relative to a fixation target viewed and extinguished prior to the saccade. From these reports, PSEs (points of subjective equality) were calculated for the fixation target as measured by the test [lashes. The distance of the trigger point from the previous fixation position was systematically varied in each experiment. Different experiments required saccades of different lengths and directions. With the exception of the presentation of the test [lash the saccades were carried out in complete darkness so that the possible utilization of an extraretinal signal regarding the eye movement (change in eye position, the intention to turn the eye, or a change of attention related to the eye movement) in the determination of visual direction could be observed uncomplicated by a continuing visual context. According to classical theories, an extraretinal signal proportional to the change in eye position acts to maintain direction constancy by compensating for the Shift of the retinal image resulting from the movement of the eye. In general, direction constancy was not preserved in the present experiments, and thus the data would not be predicted by classical theories. However, the PSE varied with distance of the trigger point from the fixation target. Since this displacement of PSE from the trigger point was in the correct direction for compensation, the presence of an extraretinal signal was confirmed. However, the growth of this signal appears to be time-locked to the saccade rather than locked to eye position; it is suggested that this growth takes place over a time period which is longer than the duration of the saccade itself.

Visual perception of direction when voluntary saccades occur: II. Relation of visual direction of a fixation target extinguished before a saccade to a subsequent test flash presented before the saccade

Previously reported experiments demonstrated changes in the relation of visual direction to retinal locus for stimulation during voluntary saccades as compared to this relation before saccade initiation. The quantitative features of these results led to the prediction, confirmed in the present experiments, that there are shifts in visual direction for stimulation presented before the saccade itself. In the present report, monotonically increasing shifts were mapped with stimuli presented as early as 240 msec before the saccade up to the saccade itself. Such shifts cannot be accounted for readily by “inflowing” processes, and while “outflowing” processes seem to be implicated, their quantitative characteristics would need to be considerably different from those required by classical outflow theories.

Eye movements in the dark during the attempt to maintain a prior fixation position

Abstract The horizontal eye movements of two subjects were measured during a series of 3-sec intervals of total darkness as they attempted to maintain the ocular position defined by prior viewing of a fixation target. Systematic changes in the mean and variance (over trials) of displacements of the eye occurred between successive portions of the dark interval. The pattern of correlations between eye position at a given moment in the dark interval and subsequent displacements contained statistically significant deviations from expectation based on a random walk model indicating some control of eye position by an extraretinal signal; however, these deviations were not large. The extraretinal signal is itself often in error with regard to the position toward which correction does take place. It is suggested that the extraretinal signal is the subjects perception of a discrepancy between the effort pattern exerted in “attempting to hold the same position” and his memory for what this effort pattern was during fixation of the visible target; the ocular displacements in the dark are a manifestation of the noisiness of this signal.

Visual perception of direction in the dark: Roles of local sign, eye movements, and ocular proprioception

Abstract Subjects performing monocularly in an otherwise dark room reported the direction at which a flash (6 msec duration, 3.5 min visual angle, randomly located along the horizontal in the frontal parallel plane) appeared relative to a fixation target extinguished 3 sec earlier. Although the subjects attempted to maintain the eye in the same position as during the prior fixation period, large involuntary eye movements (monitored by a contact-lens technique) during the 3 sec dark interval caused a given flash target to strike the retina to the left of the fovea on some trials, and to the right on others. The report of flash direction depended strongly on the sign and magnitude of this varying retinal signal independently of the physical location of the flash target. The standard deviation of the function relating report of flash direction to the retinal signal was approximately half of the standard deviation of the function relating the report to physical target location. No evidence was found that proprioceptive signals regarding the eye movements systematically influenced the reports of flash direction. The accuracy of the report of the physical location of the target was thus limited by the subjects ability to maintain his eye close to the original fixation position.

Acuity for orientation measured with a sequential recognition task and signal detection methods

An orientation recognition task with two stimuli was employed as the basis of a psychophysical method for measuring sensitivity to orientation. Essentially, this method depends on a subject’s ability to discriminate between two stimuli differing in orientation and presented to the same retinal area in random succession from trial to trial. Advantages of this sequential method over traditional simultaneous matching methods are discussed. Its feasibility is demonstrated in a signal detection analysis of acuity for the orientation of short luminous slits presented foveally on a dark field at seven reference orientations varying between 0° and 90°. In both subjects employed, sensitivity was greater for horizontal and vertical orientations than for other slopes (the oblique effect).

Light adaptation and the dynamics of induced tilt

Abstract The dynamic properties of induced tilt were measured with a temporal paradigm similar to the ones employed for studies of changes in sensitivity during early light and dark adaptation. A single vertical inducing line was presented for a period of 3 sec/trial in two experiments and as a 10 msec flash in a third experiment. At varying times either before, during or after its presentation, a slanted test line was flashed for 10 msec. Its top terminated at the left edge of the middle of the inducing line and formed an angle of 30° with it. The perceived tilt of this test line changed when it was presented in conjunction with the inducing line, by an amount which depended on the time when the flash occurred. Psychophysical measurements with test and inducing stimuli of the same luminance revealed that the change in perceived tilt precedes the onset of the inducing line by 100–200 msec (backward tilt induction), comes to a maximum at the onset of the inducing line, and then decreases. When the inducing line is presented for 3 sec, a second relative maximum occurs near its offset prior to the eventual decay during the tilt aftereffect period. The discussion relates the results to current theories which attribute the tilt illusion and aftereffect to adaptation or lateral inhibition in cortical tilt detectors and suggests an alternative to them. The significance of the resemblance between the temporal properties of induced tilt and of masking is considered.

Vernier discrimination with sequentially-flashed lines: Roles of eye movements, retinal offsets and short-term memory

The two lines of a vertical vernier target were sequentially flashed (2 msec/line; 0.2–800 msec dark interval) while the subject tried to maintain an earlier position of fixation in total darkness. Vernier discrimination deteriorates with increasing dark interval: acuity threshold increases from 29″–26′; constant error generally increases. The errors are a joint result of: (1) displacements between the retinal images of the two lines that are not present in the target; these are produced by involuntary eye movements in the dark interval and (2) deterioration of memory for spatial location signalled by the first line flashed. The entire contribution of involuntary eye movements is due to its effect on retinal offset; extraretinal signals related to involuntary changes in eye position during the dark interval do not influence the discrimination at any dark interval. A systematic influence of eye-movement-produced retinal offset is measured even for the shortest dark interval of 0.2 msec (2.2 msec between centers of flashes). Memory deterioration predominates in determining both constant and variable errors with brief dark intervals; eye-movement-produced retinal offset increases in significance at longer ones. The course of memory deterioration is consistent with a random walk model. At the shortest dark interval acuity was affected by the vertical separation between the two lines of the vernier target; at longer dark intervals this influence was obscured by the eye-movement-produced increases in retinal distance and memory deterioration.

Information transfer rate with serial and simultaneous visual display formats

Information communication rate for a conventional display with three spatially separated windows was compared with rate for a serial display in which data frames were presented sequentially in one window. For both methods, each frame contained a randomly selected digit with various amounts of additional display “clutter.” Subjects recalled the digits in a prescribed order. Large rate differences were found, with faster serial communication for all levels of the clutter factors. However, the rate difference was most pronounced for highly cluttered displays. An explanation for the latter effect in terms of visual masking in the retinal periphery was supported by the results of a second experiment. The working hypothesis that serial displays can speed information transfer for automatic but not for controlled processing is discussed.

Orientation discrimination as a function of orientation and spatial frequency

Orientation discrimination was measured with a yes-no signal detection procedure at two reference orientations, 90° and 45°. For the first of two experiments, the stimuli were disk-shaped gratings, 2.5° in diameter. Their spatial frequencies ranged from 1.8 cpd to 17.6 cpd. For the second experiment, the stimuli were either gratings or bars consisting of the center cycle of the corresponding grating. As expected, the results showed a highly significant reference orientation factor, with smaller just noticeable differences at 90° than at 45° (theoblique effect). However, orientation discrimination was independent of spatial frequency and bar width. Implications of the findings are discussed in relation to a holographic model that relates the oblique effect to meridional differences in the number of orientation-tuned cortical cells.