Biomedical subjects

Shaul Hochstein

Publications and source records attributed to Shaul Hochstein.

9 recordsLinked to original sources

PubMed2006

Eye dominance effects in feature search.

We studied the role of eye dominance in non-rivalry conditions, testing dichoptic visual search and comparing performance with target presented to the dominant or non-dominant eye. Using red-green glasses, subjects viewed an array of green and red lines of uniform orientation, with a differently oriented target line present on half the trials. Performance was significantly better when the dominant eye saw the target, especially when the opposite eye saw the distractors. This effect was reduced when only nearest-neighbor surrounding distractors were homogeneous. We conclude that the dominant eye has priority in visual processing, perhaps including inhibition of non-dominant eye representations.

Adult↗

PubMed2006

With a careful look: still no low-level confound to face pop-out.

In this issue of Vision Research, VanRullen, R. (2006). On second glance: Still no high-level pop-out effect for faces. Vision Research, in press. challenges our earlier Vision Research paper, "At first sight: A high-level pop-out effect for faces" (Hershler, O., & Hochstein, S. (2005). At first sight: A high-level pop-out effect for faces. Vision Research, 45, 1707-1724). In that paper, we showed that faces pop-out from a great variety of heterogeneous distractors. This search must have been based on a holistic combination of facial features, since it could not have relied on any single low-level distinguishing feature-each of which was present in at least some of the distractors. VanRullen implies that the pop-out effect is not limited to faces, is not holistic, and is due to a low-level confound, namely that the "low-level" Fourier amplitude spectrum may differentiate between faces and other categories. We now show that he fails to substantiate all three claims. His first experiment replicates our own and shows once again that faces do indeed pop-out, while other objects, such as cars, do not. The claim regarding the non-holistic nature of face search is based on a failure to differentiate between holistic processing for face detection and for individual face identification. His central claim is that the Fourier amplitude spectrum is processed low-level and could be used for face pop-out. However, changing the amplitude spectrum may well affect high-level representations as well. For example, his demonstration uses hybrid images which are extremely fuzzy, rendering them difficult to identify. More importantly, this claim would lead to the conclusion that targets with a non-face phase spectrum and only a face amplitude spectrum would pop-out among distractors with different amplitude spectra. We demonstrate that this is, of course, not the case and that the Fourier amplitude is not the hoped for "low-level confound". Until another such "hidden" low level feature is found, we must accept that face pop out depends on a high level mechanism.

Discrimination, Psychological↗

PubMed2006

Memory of ordinal number categories in macaque monkeys.

What mechanism underlies serial order memory? Studying preverbal serial memory shows that macaque monkeys reproducing a sequence of items can acquire knowledge of item ordinal position. In our previous experiment, macaques were repeatedly presented with image lists (first shown sequentially and then simultaneously on a touch screen together with a distractor chosen randomly from other lists). The task was to touch list images in the correct order. The monkeys' natural tendency was to categorize images by their ordinal position or number because their most common error was touching the distractor when it had the same ordinal number (in its own list) as the correct image. Item-to-item associations were used to complete the categorization strategy. Proposing a dynamic image-salience hypothesis for serial recall (based on category-to-image influence and a salience computation for identifying touch targets), we now study the category label characteristics in the context of this hypothesis. We found that these category labels are absolute, ordinal-number-based categories (first, second, etc.), not relative memorized as relative distance from the beginning and the end of the list, and not based on fixed ranking of reward contingency/image familiarity. Even isolated from item-item associations, the categories demonstrate category tuning (as well as the corresponding overlap of adjacent ordinal number codes). Moreover, monkeys choose images by proximity of their category to the current touch number, irrespective of the accuracy of the preceding choice. Category tuning itself is symmetric relative to correct ordinal position, but is skewed by other factors (reward, etc.). Tuning width increases with list length, with a concurrent increased use of item-to-item associations for determining touch order.

Animals↗

PubMed2005

At first sight: a high-level pop out effect for faces.

To determine the nature of face perception, several studies used the visual search paradigm, whereby subjects detect an odd target among distractors. When detection reaction time is set-size independent, the odd element is said to "pop out", reflecting a basic mechanism or map for the relevant feature. A number of previous studies suggested that schematic faces do not pop out. We show that natural face stimuli do pop out among assorted non-face objects. Animal faces, on the other hand, do not pop out from among the same assorted non-face objects. In addition, search for a face among distractors of another object category is easier than the reverse search, and face search is mediated by holistic face characteristics, rather than by face parts. Our results indicate that the association of pop out with elementary features and lower cortical areas may be incorrect. Instead, face search, and indeed all feature search, may reflect high-level activity with generalization over spatial and other property details.

Animals↗

PubMed2004

Multi-item working memory--a behavioral study.

Macaque monkeys were trained to recognize the repetition of one of the images already seen in a sequence of random length. On average, performance decreased with sequence length. However, this was due to a complex combination of factors, as follows: performance was found to decrease with the separation in the sequence of the test (repetition image) from the cue (its first appearance in the sequence), for trials with sequences of fixed length. In contrast, performance improved as a function of sequence length, for equal cue-test separations. Reaction times followed a complementary trend: they increased with cue-test separation and decreased with sequence length. The frequency of false positives (FPs) indicates that images are not always removed from working memory between successive trials, and that the monkeys rarely confuse different images. The probability of miss errors depends on number of intervening stimulus presentations, while FPs depend on elapsed time. A simple two-state stochastic model of multi-item working memory is proposed that guides the account for the main effects of performance and false positives, as well as their interaction. In the model, images enter WM when they are presented, or by spontaneous jump-in. Misses are due to spontaneous jump-out of images previously seen.

Animals↗

PubMed2004

The reverse hierarchy theory of visual perceptual learning.

Perceptual learning can be defined as practice-induced improvement in the ability to perform specific perceptual tasks. We previously proposed the Reverse Hierarchy Theory as a unifying concept that links behavioral findings of visual learning with physiological and anatomical data. Essentially, it asserts that learning is a top-down guided process, which begins at high-level areas of the visual system, and when these do not suffice, progresses backwards to the input levels, which have a better signal-to-noise ratio. This simple concept has proved powerful in explaining a broad range of findings, including seemingly contradicting data. We now extend this concept to describe the dynamics of skill acquisition and interpret recent behavioral and electrophysiological findings.

Cerebral Cortex↗

PubMed2002

View from the top: hierarchies and reverse hierarchies in the visual system.

We propose that explicit vision advances in reverse hierarchical direction, as shown for perceptual learning. Processing along the feedforward hierarchy of areas, leading to increasingly complex representations, is automatic and implicit, while conscious perception begins at the hierarchy's top, gradually returning downward as needed. Thus, our initial conscious percept--vision at a glance--matches a high-level, generalized, categorical scene interpretation, identifying "forest before trees." For later vision with scrutiny, reverse hierarchy routines focus attention to specific, active, low-level units, incorporating into conscious perception detailed information available there. Reverse Hierarchy Theory dissociates between early explicit perception and implicit low-level vision, explaining a variety of phenomena. Feature search "pop-out" is attributed to high areas, where large receptive fields underlie spread attention detecting categorical differences. Search for conjunctions or fine discriminations depends on reentry to low-level specific receptive fields using serial focused attention, consistent with recently reported primary visual cortex effects.

Humans↗

PubMed2002

Searching with unilateral neglect.

We address two longstanding conflicts in the visual search and unilateral neglect literature by studying feature and conjunction search performance of neglect patients using laterally presented search arrays. The first issue relates to whether feature search is performed independently of attention, or rather requires "spread attention." If feature search is "preattentive," it should survive neglect. However, we find neglect effects for both feature and conjunction search, suggesting that feature search, too, has an attentional requirement. The second controversy refers to the space- or object-based nature of neglect following unilateral right-hemisphere parietal lobe damage. If neglect were a purely spatial phenomenon, then we would expect no detriment in performance in the right (nonneglect) field, and diminished performance for the whole left (neglect) field. On the other hand, if neglect were purely object-based, we would expect diminished performance on the left side of the search array, irrespective of its location in the visual field. We now demonstrate a combination of strong object-based and space-based neglect effects for conjunction search with laterally placed element arrays, suggesting that these two mechanisms work in tandem.

Adult↗

PubMed2002

Serial memory strategies in macaque monkeys: behavioral and theoretical aspects.

Serial memory is the ability to encode and retrieve a list of items in their correct temporal order. To study nonverbal strategies involved in serial memory, we trained four macaque monkeys on a novel delayed sequence-recall task and analysed the mechanisms underlying their performance in terms of a neural network model. Thirty fractal images, divided into 10 triplets, were presented repeatedly in fixed temporal order. On each trial the monkeys viewed three sequentially presented sample images, followed by a test stimulus consisting of the same triplet of images and a distractor image (chosen randomly from the remaining 27). The task was to touch the three images in their original order, avoiding the distractor. The monkeys' most common error was touching the distractor when it had the same ordinal position (in its own triplet) as the correct image. This finding suggests that monkeys naturally categorize images by their ordinal number. Additional, secondary strategies were eventually used to avoid distractor images. These include memory of the sample images (working memory) and associations between triplet members. Further direct evidence for ordinal number categorization was provided by a transfer of learning to untrained images of the same ordinal category, following reassignment of image categories within each triplet. We propose a generic three-tier neuronal framework that can explain the components and complex set of characteristics of the observed behavior. This framework, with its intermediate level representing ordinal categories, can also explain the transfer of learning following category reassignment.

Animals↗