Biomedical subjects

John M Findlay

Publications and source records attributed to John M Findlay.

13 recordsLinked to original sources

PubMed2006

Binocular coordination of the eyes during reading.

Saccadic eye movements and fixations are the behavioral means by which we visually sample text during reading. Human oculomotor control is governed by a complex neurophysiological system involving the brain stem, superior colliculus, and several cortical areas. A very widely held belief among researchers investigating primate vision is that the oculomotor system serves to orient the visual axes of both eyes to fixate the same target point in space. It is argued that such precise positioning of the eyes is necessary to place images on corresponding retinal locations, such that on each fixation a single, nondiplopic, visual representation is perceived. Vision works actively through a continual sampling process involving saccades and fixations. Here we report that during normal reading, the eyes do not always fixate the same letter within a word. We also demonstrate that saccadic targeting is yoked and based on a unified cyclopean percept of a whole word since it is unaffected if different word parts are delivered exclusively to each eye via a dichoptic presentation technique. These two findings together suggest that the visual signal from each eye is fused at a very early stage in the visual pathway, even when the fixation disparity is greater than one character (0.29 deg), and that saccade metrics for each eye are computed on the basis of that fused signal.

Fixation, Ocular↗

PubMed2006

The binocular coordination of eye movements during reading in children and adults.

Recent evidence indicates that each eye does not always fixate the same letter during reading and there has been some suggestion that processing difficulty may influence binocular coordination. We recorded binocular eye movements from children and adults reading sentences containing a word frequency manipulation. We found disparities of significant magnitude between the two eyes for all participants, with greater disparity magnitudes in children than adults. All participants made fewer crossed than uncrossed fixations. However, children made a higher proportion of crossed fixations than adults. We found no influence of word frequency on children's fixations and on binocular coordination in adults.

Adolescent↗

PubMed2006

Binocular coordination of eye movements during reading.

Binocular coordination of the eyes during reading was examined. Fixation disparity greater than one character occurred on 47% of fixations, with the disparity being predominantly uncrossed (39%), though a small proportion of fixations were crossed. The average magnitude of disparity, measured at the end of fixation, was 1.1 characters for all fixations. For the 47% of non-aligned fixations the average magnitude of disparity was 1.9 characters. Vergence movements that reduced fixation disparity occurred during fixations, and their magnitude was positively correlated with fixation duration. Finally, eye dominance did not modulate fixation disparity magnitude or the proportion of disparate fixations.

Dominance, Ocular↗

PubMed2006

Between-object and within-object saccade programming in a visual search task.

The role of the perceptual organization of the visual display on eye movement control was examined in two experiments using a task where a two-saccade sequence was directed toward either a single elongated object or three separate shorter objects. In the first experiment, we examined the consequences for the second saccade of a small displacement of the whole display during the first saccade. We found that between-object saccades compensated for the displacement to aim for a target position on the new object whereas within-object saccades did not show compensation but were coded as a fixed motor vector applied irrespective of wherever the preceding saccade landed. In the second experiment, we extended the paradigm to examine saccades performed in different directions. The results suggest that the within-object and between-object saccade distinction is an essential feature of saccadic planning.

Adult↗

PubMed2005

Eye scanning of multi-element displays: II. Saccade planning.

The properties of saccadic eye movements were studied in a task that required observers to scan through a display consisting of a set of discrete objects. The saccades forming the scanpath showed very high accuracy with almost no undershoot provided no distractor item was located within a critical region around the saccade target. When a distractor item was located in the critical region, saccade accuracy was impaired. This result suggests that the intrinsic spatial selection process for saccadic scanning movements is of low resolution. The initial saccade following display onset showed different properties. The instructions required the observer to look first at a clearly distinct target at the top left of the display. However, the first saccade frequently showed 'oculomotor capture' and was directed either to a different item in the display, or to a midway location between items. The fixation following such erroneous first saccades was generally very short and showed a substantial drift towards the instructed location.

Adult↗

PubMed2005

Eye scanning of multi-element displays: I. Scanpath planning.

We recorded oculomotor scanpaths in a task that required individuals to scan through displays consisting of a small number (between 3 and 12) of near-identical items. The task required each item to be fixated at least once and our objective was to explore the principles governing the generation of scanpaths. In general the observers carried out the task efficiently, although omissions occurred quite frequently (about 25% of trials) in the 12-item case. Backtracking occurred rarely except in the case of immediate rescanning back to the previously fixated item. Such immediate backtracking occurred on about 4% of fixations and, in contrast to more distant backtracking, was not associated with increased errors. Evidence was found for both directional (raster-like) scanning strategies and scanning strategies based on the global external contour.

Adult↗

PubMed2004

Object structure and saccade planning.

When orienting to a newly appearing display, evidence shows that two saccadic eye movements are often prepared together. By looking the relation between the landing positions of the first and the second saccade, we examine the frame of reference used for the preparation of the second saccade aiming for a new object or exploring within the same object. We demonstrate that the action to be performed on the object affects the coding of the second saccade. A second saccade directed to a new object is coded to aim for a target position on it and is adjusted to the landing position of the first saccade, whereas a second saccade within the same object is coded as a fixed motor vector applied irrespective of the initial landing position on the object.

Attention↗

PubMed2004

Eye movements when reading disappearing text: is there a gap effect in reading?

Readers' eye movements were monitored when they read either normal sentences or sentences with masked or disappearing text (in which the fixated word disappeared or was masked after 60 ms). The goals of the research were to investigate (1) whether a gap effect occurred in reading and (2) the influence of linguistic and visual factors on oculomotor control. The results of a number of global analyses of eye movements under disappearing text conditions clearly demonstrated that there is no gap effect in reading. However, comparative analyses across a number of local measures in the experiments indicated that cognitive/lexical processes, as well as the continual uptake of visual information, influence eye movement control during reading. A persistent visual object throughout fixation caused refixations and even when a fixated word had disappeared (or been masked), there were significant effects of word frequency and word length.

Cognition↗

PubMed2004

Is object search mediated by object-based or image-based representations?

Recent research suggests that visually specific memory representations for previously fixated objects are maintained during scene perception. Here we investigate the degree of visual specificity by asking whether the memory representations are image-based or object-based. To that end we measured the effects of object orientation on the time to search for a familiar object from amongst a set of 7 familiar distractors arranged in a circular array. Search times were found to depend on the relative orientations of the target object and the probe object for both familiar and novel objects. This effect was found to be partly an image matching effect but there was also an advantage shown for the object's canonical view for familiar objects. Orientation effects were maintained even when the target object was specified as having unique or similar shape properties relative to the distractors. Participants' eye movements were monitored during two of the experiments. Eye movement patterns revealed selection for object shape and object orientation during the search process. Our findings provide evidence for object representations during search that are detailed and share image-based characteristics with more high-level characteristics from object memory.

Adult↗

PubMed2003

Saccade target selection in visual search: accuracy improves when more distractors are present.

We report four experiments with search displays of Gabor patches. Our aim was to study the accuracy of gaze control in search tasks. In Experiment 1, a target was presented with a single distractor Gabor of a different spatial frequency on the same axis. Subjects could locate the target with the first saccade if the distractor was more distant, but when the distractor was between the fixation point and the target, the first saccade landed much closer to the distractor. In Experiment 2, the number of display items was increased to 16 in a double ring configuration. With this configuration, first saccades were accurately directed to the target, even when there was an intervening distractor in exactly the same configuration as in Experiment 1. Experiment 3 suggested that the improvement in accuracy was not due to distractor homogeneity but rather may be attributable to the increased first saccade latency with the ring configuration. In the final experiment, latency was shown to covary with saccade accuracy. The results are related to a general framework whereby the presence of distractors operates to hold fixation for a longer period of time, thus allowing a greater period of visual processing and more accurate eye movements.

Adult↗

PubMed2003

Foveal stimulation and saccadic latencies.

We investigated how a postsaccadic visual error affects the latency of the subsequent saccade. Observers made sequences of saccades directed to a single object or three separate objects, which could be displaced backward or onward during the primary saccade. The results showed an increase of second saccade latency after a backward displacement and a decrease of second saccade latency after an onward displacement. Moreover, second saccade latencies were particularly short after an onward displacement which resulted in a primary saccade landing position outside the boundary of the objects. We interpret the latency differences in terms of known effects within the saccadic system, specifically the sensitivity to the pattern of visual stimulation, particularly at and near the fovea, and the inverse correlation between secondary saccade amplitudes and secondary saccade latencies.

Fovea Centralis↗

PubMed2003

Visual sensitivity in search tasks depends on the response requirement.

This paper reports a comparison between two tasks of visual search. Two observers carried out, in separate blocks, a saccade-to-target task and a manual-target-detection task. The displays, which were identical for the two tasks, consisted of a ring of eight equally spaced Gabor patches. The target could be defined by a difference from the distractors along four possible dimensions: orientation, spatial frequency, contrast or size. These four dimensions were used as variables in separate experiments. In each experiment, performance was measured over an extensive range of values of the particular dimension. Thresholds were thus obtained for the saccade and the manual response tasks. The nature of the response was found to modify the relative visual sensitivity. For orientation differences, manual response performance was better than saccade-to-target performance. The reverse was true for spatial frequency and contrast differences, where saccade-to-target performance was better than manual response performance. We conclude that saccade-selection in a search task draws on different visual information from that used for manual responding in the equivalent task. The two tasks thus differ in more than the different response systems used: the results suggest the action of different underlying neural visual mechanisms as well as different neural motor mechanisms.

Contrast Sensitivity↗

PubMed2002

Are spatial frequencies integrated from coarse to fine?

The existence of a temporal anisotropy in the integration of spatial frequencies, such that spatial frequencies are integrated more effectively if they are available from low to high through time, has been examined in a series of experiments. In the first experiment, the first three harmonics of a square wave were presented in a low-to-high or a high-to-low sequence in a temporal two-interval forced-choice experiment. Subjects were asked to indicate which sequence appeared to resemble a square wave more. A high-to-low sequence of spatial frequencies was judged to more resemble the target than the low-to-high sequence. These results support a temporal anisotropy in the integration of spatial frequencies of exactly the opposite form to that suggested from previous results. Further experiments established that this was not due to task differences or to subjects basing their decision on the final spatial frequency shown. An interpretation is offered in which an isotropic mechanism for spatial-frequency integration is combined with a recency bias.

Adult↗