The existence and role of retinotopic and spatiotopic forms of visual persistence

The perceptual timescape: Perceptual history on the sub-second scale

There is a high-capacity store of brief time span (∼1000 ms) which information enters from perceptual processing, often called iconic memory or sensory memory. It is proposed that a main function of this store is to hold recent perceptual information in a temporally segregated representation, named the perceptual timescape. The perceptual timescape is a continually active representation of change and continuity over time that endows the perceived present with a perceived history. This is accomplished primarily by two kinds of time marking information: time distance information, which marks all items of information in the perceptual timescape according to how far in the past they occurred, and ordinal temporal information, which organises items of information in terms of their temporal order. Added to that is information about connectivity of perceptual objects over time. These kinds of information connect individual items over a brief span of time so as to represent change, persistence, and continuity over time. It is argued that there is a one-way street of information flow from perceptual processing either to the perceived present or directly into the perceptual timescape, and thence to working memory. Consistent with that, the information structure of the perceptual timescape supports postdictive reinterpretations of recent perceptual information. Temporal integration on a time scale of hundreds of milliseconds takes place in perceptual processing and does not draw on information in the perceptual timescape, which is concerned with temporal segregation, not integration.

Neural correlates of perisaccadic visual mislocalization in extrastriate cortex

When interacting with the visual world using saccadic eye movements (saccades), the perceived location of visual stimuli becomes biased, a phenomenon called perisaccadic mislocalization, which is indeed an exemplar of the brain's dynamic representation of the visual world. However, the neural mechanism underlying this altered visuospatial perception and its potential link to other perisaccadic perceptual phenomena have not been established. Using a combined experimental and computational approach, we were able to quantify spatial bias around the saccade target (ST) based on the perisaccadic dynamics of extrastriate spatiotemporal sensitivity captured by statistical models. This approach could predict the perisaccadic spatial bias around the ST, consistent with the psychophysical studies, and revealed the precise neuronal response components underlying representational bias. These findings also established the crucial role of response remapping toward ST representation for neurons with receptive fields far from the ST in driving the ST spatial bias. Moreover, we showed that, by allocating more resources for visual target representation, visual areas enhance their representation of the ST location, even at the expense of transient distortions in spatial representation. This potential neural basis for perisaccadic ST representation, also supports a general role for extrastriate neurons in creating the perception of stimulus location.

A mathematical model of local and global attention in natural scene viewing

Understanding the decision process underlying gaze control is an important question in cognitive neuroscience with applications in diverse fields ranging from psychology to computer vision. The decision for choosing an upcoming saccade target can be framed as a selection process between two states: Should the observer further inspect the information near the current gaze position (local attention) or continue with exploration of other patches of the given scene (global attention)? Here we propose and investigate a mathematical model motivated by switching between these two attentional states during scene viewing. The model is derived from a minimal set of assumptions that generates realistic eye movement behavior. We implemented a Bayesian approach for model parameter inference based on the model's likelihood function. In order to simplify the inference, we applied data augmentation methods that allowed the use of conjugate priors and the construction of an efficient Gibbs sampler. This approach turned out to be numerically efficient and permitted fitting interindividual differences in saccade statistics. Thus, the main contribution of our modeling approach is two-fold; first, we propose a new model for saccade generation in scene viewing. Second, we demonstrate the use of novel methods from Bayesian inference in the field of scan path modeling.

The perceived present: What is it, and what is it there for?

It is proposed that the perceived present is not a moment in time, but an information structure comprising an integrated set of products of perceptual processing. All information in the perceived present carries an informational time marker identifying it as "present". This marker is exclusive to information in the perceived present. There are other kinds of time markers, such as ordinality ("this stimulus occurred before that one") and duration ("this stimulus lasted for 50 ms"). These are different from the "present" time marker and may be attached to information regardless of whether it is in the perceived present or not. It is proposed that the perceived present is a very short-term and very high-capacity holding area for perceptual information. The maximum holding time for any given piece of information is ~100 ms: This is affected by the need to balance the value of informational persistence for further processing against the problem of obsolescence of the information. The main function of the perceived present is to facilitate access by other specialized, automatic processes.

Bridgemanian space constancy as a precursor to extended cognition

A few decades ago, cognitive psychologists generally took for granted that the reason we perceive our visual environment as one contiguous stable whole (i.e., space constancy) is because we have an internal mental representation of the visual environment as one contiguous stable whole. They supposed that the non-contiguous visual images that are gathered during the brief fixations that intervene between pairs of saccadic eye movements (a few times every second) are somehow stitched together to construct this contiguous internal mental representation. Determining how exactly the brain does this proved to be a vexing puzzle for vision researchers. Bruce Bridgeman's research career is the story of how meticulous psychophysical experimentation, and a genius theoretical insight, eventually solved this puzzle. The reason that it was so difficult for researchers to figure out how the brain stitches together these visual snapshots into one accurately-rendered mental representation of the visual environment is that it doesn't do that. Bruce discovered that the brain couldn't do that if it tried. The neural information that codes for saccade amplitude and direction is simply too inaccurate to determine exact relative locations of each fixation. Rather than the perception of space constancy being the result of an internal representation, Bruce determined that it is the result of a brain that simply assumes that external space remains constant, and it rarely checks to verify this assumption. In our extension of Bridgeman's formulation, we suggest that objects in the world often serve as their own representations, and cognitive operations can be performed on those objects themselves, rather than on mental representations of them.

The influence of scene context on parafoveal processing of objects

Many studies in reading have shown the enhancing effect of context on the processing of a word before it is directly fixated (parafoveal processing of words). Here, we examined whether scene context influences the parafoveal processing of objects and enhances the extraction of object information. Using a modified boundary paradigm called the Dot-Boundary paradigm, participants fixated on a suddenly onsetting cue before the preview object would onset 4° away. The preview object could be identical to the target, visually similar, visually dissimilar or a control (black rectangle). The preview changed to the target object once a saccade toward the object was made. Critically, the objects were presented on either a consistent or an inconsistent scene background. Results revealed that there was a greater processing benefit for consistent than inconsistent scene backgrounds and that identical and visually similar previews produced greater processing benefits than other previews. In the second experiment, we added an additional context condition in which the target location was inconsistent, but the scene semantics remained consistent. We found that changing the location of the target object disrupted the processing benefit derived from the consistent context. Most importantly, across both experiments, the effect of preview was not enhanced by scene context. Thus, preview information and scene context appear to independently boost the parafoveal processing of objects without any interaction from object–scene congruency.

Time-Order Errors in Duration Judgment Are Independent of Spatial Positioning

Time-order errors (TOEs) occur when the discriminability between two stimuli are affected by the order in which they are presented. While TOEs have been studied since the 1860s, it is unknown whether the spatial properties of a stimulus will affect this temporal phenomenon. In this experiment, we asked whether perceived duration, or duration discrimination, might be influenced by whether two intervals in a standard two-interval method of constants paradigm were spatially overlapping in visual short-term memory. Two circular sinusoidal gratings (one standard and the other a comparison) were shown sequentially and participants judged which of the two was presented for a longer duration. The test stimuli were either spatially overlapping (in different spatial frames) or separate. Stimulus order was randomized between trials. The standard stimulus lasted 600 ms, and the test stimulus had one of seven possible values (between 300 and 900 ms). There were no overall significant differences observed between spatially overlapping and separate stimuli. However, in trials where the standard stimulus was presented second, TOEs were greater, and participants were significantly less sensitive to differences in duration. TOEs were also greater in conditions involving a saccade. This suggests there is an intrinsic memory component to two interval tasks in that the information from the first interval has to be stored; this is more demanding when the standard is presented in the second interval. Overall, this study suggests that while temporal information may be encoded in some spatial form, it is not dependent on visual short-term memory.

A New Conceptualization of Human Visual Sensory-Memory

Memory is an essential component of cognition and disorders of memory have significant individual and societal costs. The Atkinson–Shiffrin “modal model” forms the foundation of our understanding of human memory. It consists of three stores: Sensory Memory (SM), whose visual component is called iconic memory, Short-Term Memory (STM; also called working memory, WM), and Long-Term Memory (LTM). Since its inception, shortcomings of all three components of the modal model have been identified. While the theories of STM and LTM underwent significant modifications to address these shortcomings, models of the iconic memory remained largely unchanged: A high capacity but rapidly decaying store whose contents are encoded in retinotopic coordinates, i.e., according to how the stimulus is projected on the retina. The fundamental shortcoming of iconic memory models is that, because contents are encoded in retinotopic coordinates, the iconic memory cannot hold any useful information under normal viewing conditions when objects or the subject are in motion. Hence, half-century after its formulation, it remains an unresolved problem whether and how the first stage of the modal model serves any useful function and how subsequent stages of the modal model receive inputs from the environment. Here, we propose a new conceptualization of human visual sensory memory by introducing an additional component whose reference-frame consists of motion-grouping based coordinates rather than retinotopic coordinates. We review data supporting this new model and discuss how it offers solutions to the paradoxes of the traditional model of sensory memory.

The Role of Fixation Position in Detecting Scene Changes Across Saccades

Target objects presented within color images of naturalistic scenes were deleted or rotated during a saccade to or from the target object or to a control region of the scene. Despite instructions to memorize the details of the scenes and to monitor for object changes, viewers frequently failed to notice the changes. However, the failure to detect change was mediated by three other important factors: First, accuracy generally increased as the distance between the changing region and the fixation immediately before or after the change decreased. Second, changes were sometimes initially missed, but subsequently noticed when the changed region was later refixated. Third, when an object disappeared from a scene, detection of that disappearance was greatly improved when the deletion occurred during the saccade toward that object. These results suggest that fixation position and saccade direction play an important role in determining whether changes will be detected. It appears that more information can be retained across views than has been suggested by previous studies.

Transsaccadic processing: stability, integration, and the potential role of remapping

While our frequent saccades allow us to sample the complex visual environment in a highly efficient manner, they also raise certain challenges for interpreting and acting upon visual input. In the present, selective review, we discuss key findings from the domains of cognitive psychology, visual perception, and neuroscience concerning two such challenges: (1) maintaining the phenomenal experience of visual stability despite our rapidly shifting gaze, and (2) integrating visual information across discrete fixations. In the first two sections of the article, we focus primarily on behavioral findings. Next, we examine the possibility that a neural phenomenon known as predictive remapping may provide an explanation for aspects of transsaccadic processing. In this section of the article, we delineate and critically evaluate multiple proposals about the potential role of predictive remapping in light of both theoretical principles and empirical findings.

Evaluating the influence of motor control on selective attention through a stochastic model: the paradigm of motor control dysfunction in cerebellar patient

Attention allows us to selectively process the vast amount of information with which we are confronted, prioritizing some aspects of information and ignoring others by focusing on a certain location or aspect of the visual scene. Selective attention is guided by two cognitive mechanisms: saliency of the image (bottom up) and endogenous mechanisms (top down). These two mechanisms interact to direct attention and plan eye movements; then, the movement profile is sent to the motor system, which must constantly update the command needed to produce the desired eye movement. A new approach is described here to study how the eye motor control could influence this selection mechanism in clinical behavior: two groups of patients (SCA2 and late onset cerebellar ataxia LOCA) with well-known problems of motor control were studied; patients performed a cognitively demanding task; the results were compared to a stochastic model based on Monte Carlo simulations and a group of healthy subjects. The analytical procedure evaluated some energy functions for understanding the process. The implemented model suggested that patients performed an optimal visual search, reducing intrinsic noise sources. Our findings theorize a strict correlation between the "optimal motor system" and the "optimal stimulus encoders."

Visual attention and stability

In the present review, we address the relationship between attention and visual stability. Even though with each eye, head and body movement the retinal image changes dramatically, we perceive the world as stable and are able to perform visually guided actions. However, visual stability is not as complete as introspection would lead us to believe. We attend to only a few items at a time and stability is maintained only for those items. There appear to be two distinct mechanisms underlying visual stability. The first is a passive mechanism: the visual system assumes the world to be stable, unless there is a clear discrepancy between the pre- and post-saccadic image of the region surrounding the saccade target. This is related to the pre-saccadic shift of attention, which allows for an accurate preview of the saccade target. The second is an active mechanism: information about attended objects is remapped within retinotopic maps to compensate for eye movements. The locus of attention itself, which is also characterized by localized retinotopic activity, is remapped as well. We conclude that visual attention is crucial in our perception of a stable world.

Location and identity memory of saccade targets

While the memory of objects' identity and of their spatiotopic location may sustain transsaccadic spatial constancy, the memory of their retinotopic location may hamper it. Is it then true that saccades perturb retinotopic but not spatiotopic memory? We address this issue by assessing localization performances of the last and of the penultimate saccade target in a series of 2-6 saccades. Upon fixation, nine letter-pairs, eight black and one white, were displayed at 3° eccentricity around fixation within a 20° × 20° grey frame, and subjects were instructed to saccade to the white letter-pair; the cycle was then repeated. Identical conditions were run with the eyes maintaining fixation throughout the trial but with the grey frame moving so as to mimic its retinal displacement when the eyes moved. At the end of a trial, subjects reported the identity and/or the location of the target in either retinotopic (relative to the current fixation dot) or frame-based(1) (relative to the grey frame) coordinates. Saccades degraded target's retinotopic location memory but not its frame-based location or its identity memory. Results are compatible with the notion that spatiotopic representation takes over retinotopic representation during eye movements thereby contributing to the stability of the visual world as its retinal projection jumps on our retina from saccade to saccade.

Visual stability based on remapping of attention pointers

When we move our eyes, we easily keep track of where relevant things are in the world. Recent proposals link this stability to the shifting of receptive fields of neurons in eye movement and attention control areas. Reports of 'spatiotopic' visual aftereffects have also been claimed to support this shifting connectivity even at an early level, but these results have been challenged. Here, the process of updating visual location is described as predictive shifts of location 'pointers' to attended targets, analogous to predictive activation seen cross-modally. We argue that these location pointers, the core operators of spatial attention, are linked to identity information and that such a link is necessary to establish a workable visual architecture and to explain frequently reported positive spatiotopic biases.

Is there a role for extraretinal factors in the maintenance of stability in a structured environment?

The calibration solution to the stability of the world despite eye movements depends, according to Bridgeman et al., upon a combination of three factors which presumably all need to operate to achieve the goal of stability. Although the authors admit (sect. 4.3, para. 5) that the relative contributions of retinal and extraretinal factors will depend on the particular viewing situation, Figure 5 (sect. 4.3) makes it clear in its representation that the role of perceptual factors is relatively minor compared to extraretinal ones. It is with this representation that this commentary wishes to take issue, believing that it occurs as a result of some assumptions about terminology that may be ambiguous, as well as some misconceptions about the circumstances in which there is a need for stability.

A theory of visual stability across saccadic eye movements

We identify two aspects of the problem of maintaining perceptual stability despite an observer's eye movements. The first, visual direction constancy, is the (egocentric) stability of apparent positions of objects in the visual world relative to the perceiver. The second, visual position constancy, is the (exocentric) stability of positions of objects relative to each other. We analyze the constancy of visual direction despite saccadic eye movements.

Three information sources have been proposed to enable the visual system to achieve stability: the structure of the visual field, proprioceptive inflow, and a copy of neural efference or outflow to the extraocular muscles. None of these sources by itself provides adequate information to achieve visual direction constancy; present evidence indicates that all three are used.

Our final question concerns how information processing operations result in a stable world. The three traditionally suggested means have been elimination, translation, or evaluation. All are rejected. From a review of the physiological and psychological evidence we conclude that no subtraction, compensation, or evaluation need take place. The problem for which these solutions were developed turns out to be a false one. We propose a “calibration” solution: correct spatiotopic positions are calculated anew for each fixation. Inflow, outflow, and retinal sources are used in this calculation: saccadic suppression of displacement bridges the errors between these sources and the actual extent of movement.

Can we equate iconic memory with visual awareness?

Every time we look around we can see a rich and detailed world surrounding us. Nevertheless, the majority of visual information seems to slip out of our thoughts instantly. Can we still say that this fleeting percept of the entire world was a conscious percept in the first place, as Block proposes?

Eye movements and the integration of visual memory and visual perception

Because visual perception has temporal extent, temporally discontinuous input must be linked in memory. Recent research has suggested that this may be accomplished by integrating the active contents of visual short-term memory (VSTM) with subsequently perceived information. In the present experiments, we explored the relationship between VSTM consolidation and maintenance and eye movements, in order to discover how attention selects the information that is to be integrated. Specifically, we addressed whether stimuli needed to be overtly attended in order to be included in the memory representation or whether covert attention was sufficient. Results demonstrated that in static displays in which the to-be-integrated information was presented in the same spatial location, VSTM consolidation proceeded independently of the eyes, since subjects made few eye movements. In dynamic displays, however, in which the to-be-integrated information was presented in different spatial locations, eye movements were directly related to task performance. We conclude that these differences are related to different encoding strategies. In the static display case, VSTM was maintained in the same spatial location as that in which it was generated. This could apparently be accomplished with covert deployments of attention. In the dynamic case, however, VSTM was generated in a location that did not overlap with one of the to-be-integrated percepts. In order to "move" the memory trace, overt shifts of attention were required.

Eye movements and visual memory: detecting changes to saccade targets in scenes

Saccade-contingent change detection provides a powerful tool for investigating scene representation and scene memory. In the present study, critical objects presented within color images of naturalistic scenes were changed during a saccade toward or away from the target. During the saccade,the critical object was changed to another object type, to a visually different token of the same object type, or was deleted from the scene. There were three main results. First, the deletion of a saccade target was special: Detection performance for saccade target deletions was very good, and this level of performance did not decline with the amplitude of the saccade. In contrast, detection of type and token changes at the saccade target, and of all changes including deletions at a location that had just been fixated but was not the saccade target, decreased as the amplitude of the saccade increased. Second, detection performance for type and token changes, both when the changing object was the target of the saccade and when the object had just been fixated but was not the saccade target, was well above chance. Third, mean gaze durations were reliably elevated for those trials in which the change was not overtly detected. The results suggest that the presence of the saccade target plays a special role in trassaccadic integration, and together with other recent findings, suggest more generally that a relatively rich scene representation is retained across saccades and stored in visual memory.

Inhibition of attended processing during saccadic eye movements

People are unable to perform some, but not all, cognitive tasks while moving their eyes. A possible common denominator among disrupted processes is the use of attention. The present research proposes and tests an attentional suppression hypothesis to evaluate this claim. This hypothesis states that because attention is obligatorily allocated to a to-be-fixated location prior to the onset of a saccade, during saccadic events attentional resources are unavailable to direct processing associated with higher order cognitive tasks. Subjects were engaged in a task that combined saccades and shifts of attention across global and local levels of hierarchical figures. When the eyes did not move, this shift took place between stimulus presentations. When saccades intervened between the stimuli, the global-local shifts of attention were interrupted, suggesting that saccades suppress cognitive processes requiring attention.

Characterising the visual buffer: real-world evidence for overwriting early in each fixation

What happens to the pictorial content of fixations when we move our eyes? Previous studies demonstrate that observers are very poor at detecting changes in natural scenes that occur across saccades, blinks, and artificial interruptions ('change blindness'). They suggest that the visual 'snapshots' of what is on the retina during a fixation are not retained and fused over successive fixations. I find similar results when volunteers are performing the complex real-life task of making a cup of tea. Volunteers can access the snapshot of the current fixation but not those of previous fixations. I suggest that volunteers are reporting the content of a low-level visual store that holds a veridical snapshot of the current fixation, rather than the retina itself. The snapshots are not 'wiped' by the saccade and remain in the buffer until they are overwritten by a new snapshot. The overwrite occurs in an all-or-none manner and can be at any time within the first 400 ms of each new fixation, with 50% of overwrites being within the first 100 ms.

The effects of scene inversion on change blindness

In two experiments, participants searched for a difference between two views of a scene. In Experiment 1, the authors extended the change-blindness findings from previous work by R. A. Rensink, J. K. O'Regan, and J. J. Clark (1997), which used an experimenter-induced global transient, to a less artificial situation in which participants searched for a difference in a pair of photographic images presented simultaneously. To examine the idea that meaning-driven endogenous orienting was responsible for the previously observed advantage for changes in center-of-interest items, the authors inverted half of the image pairs. The advantage for center-of-interest items was replicated with upright displays, but it was completely eliminated by inversion, strongly supporting the role of meaning-driven endogenous orienting in this task. With flickering displays (Experiment 2), the center-of-interest effect was completely unaffected by inversion. The authors suggest that when change blindness is induced via flicker, scene modifications are typically found by stimulus-driven rather than by meaning-driven processes.

HIGH-LEVEL SCENE PERCEPTION

Three areas of high-level scene perception research are reviewed. The first concerns the role of eye movements in scene perception, focusing on the influence of ongoing cognitive processing on the position and duration of fixations in a scene. The second concerns the nature of the scene representation that is retained across a saccade and other brief time intervals during ongoing scene perception. Finally, we review research on the relationship between scene and object identification, focusing particularly on whether the meaning of a scene influences the identification of constituent objects.

On the relations among different measures of visible and informational persistence

We report research designed to accomplish two goals. We first consider the question, raised by Coltheart (1980) and others, of whether three measures of visible and informational persistence--performance in temporally integrating two successively presented stimuli, subjective rating of the degree to which two successively presented stimuli appear to constitute a single or a dual temporal event, and partial-report performance--all measure the same underlying mental entity. We answer this question using a superset of dissociation logic called state-trace analysis (Bamber, 1979), and within the context of a systematic empirical foundation consisting of seven closely related experiments. Our second goal is to extend and apply a theory to data acquired from our seven experiments and also to data reported by other investigators. This theory, which has been confirmed in a variety of paradigms (see Busey & Loftus, 1994) assumes that (1) the initial stages of the visual system act as a low-pass linear filter which operates on a stimulus temporal waveform to produce a sensory response; (2) instantaneous rate of acquiring information from the stimulus is jointly proportional to sensory-response magnitude and proportion of as-yet-to-be-acquired stimulus information; (3) partial-report performance is determined by total amount of acquired information; (4) the probability that two events are perceived as contemporaneous is determined by the temporal correlation of their respective information-acquisition rate functions (which is similar to a suggestion by Dixon & Di Lollo, 1994); and (5) temporal integration is successful to the degree that the two temporal events are perceived as contemporaneous. This theory was highly successful in accounting for our and other investigators' temporal-integration and completeness-rating data, and was moderately successful in accounting for partial-report data. We discuss the degree to which our three persistence measures can be united within the context of our theory; we comment on the distinction between objective and subjective measures of visible persistence; and we address the decades-old question: "What is persistence good for?"

Effects of perceptual context on transsaccadic visual matching

Transsaccadic visual matching refers to the phenomenon in which presaccadic signals at the location of the sacccade goal influence the visibility of postsaccadic test signals presented at the fovea (Jüttner & Röhler, 1993). The present study uses variations of the perceptual context, provided by the structure of the presaccadic stimulus display, to investigate the influence of spatial information, decision factors, and visual attention on this form of transsaccadic information processing. The experiments yielded the following results: First, analysis of the data in terms of signal detection theory revealed that transsaccadic visual matching manifests in a shift of decision criterion (beta) rather than in a change of sensory sensitivity (d'). The criterion shift leads to a suppression of postsaccadic stimulus information that is incompatible with presaccadic processed information. Second, the matching effect strongly depends on the structure of the presaccadic display, which suggests that mechanisms of visual attention provide the spatial binding of pre- and postsaccadic stimulus information. Third, transsaccadic matching is phenomenologically tied to the execution of the saccade and effective during a postsaccadic period extending up to 160 msec after termination of the eye movement. These findings indicate a form of context-sensitive evaluation process that could subsidize the maintenance of visual stability.

Reduced perceptual dimensionality in extrafoveal vision

The classification behaviour of human observers with respect to compound Gabor signals is tested at foveal and extrafoveal retinal positions. Classification performance is analysed in terms of a probabilistic classification model recently proposed by Rentschler, Jüttner and Caelli [(1994) Vision Research, 34, 669-687]. The analysis allows inferences about structure and dimensionality of the individual internal representations underlying the classification task and their temporal evolution during the learning process. Using this technique it is found that the internal representations of direct and eccentric viewing are intrinsically incommensurable, in the sense that extrafoveal pattern representations are characterized by a lower perceptual dimension in feature space relative to the corresponding physical input signals, whereas foveal representations are not. The observed deficits cannot be renormalized by size scaling (cortical magnification); however, they can be partially reduced by learning although the learning progress strongly depends on the observer's practice. The structural incommensurability between foveal and extrafoveal representations poses constraints on possible forms of foveal-extrafoveal interaction, which might have implications on related perceptual phenomena such as visual stability across saccadic eye movements.

Capacity limitations in memory for visual locations

This paper examines people's ability to make judgments which require them to know the relative positions of objects that are not simultaneously visible is examined. It has previously been shown that people can accurately perform such a task. The current experiments test the capacity limits for such tasks. Two experiments were conducted that required subjects to make spatial judgments based on sequences of points presented two at a time. It was shown that, whereas subjects can perform accurately when memory for a small number of dots (about four) is required, increasing the number of dots results in a radical reduction in performance. This argues against both the idea that spatial memory is based on a linguistic description and the idea that it is based on an image-like representation. Rather it appears that one can form an accurate representation of the spatial properties of a small number of objects.

Neuronal death of the cancellation theory?

The question of how the brain can construct a stable representation of the external world despite eye movements is a very old one. If there have been some wrong statements of problems (such as the inverted retinal image), other statements are less naive and have led to analytic solutions possibly adopted by the brain to counteract the spurious effects of eye movements. Following the MacKay (1973) objections to the analytic view of perceptual stability, Bridgeman et al. claim that the idea that signals canceling the effects of saccadic eye movements are needed is also a misconception, as is the claim that stability and position encoding are two distinct problems. It must be remembered, however, that what made the theory of “cancellation” formulated by von Holst and Mittelstaedt (1950) so appealing was the clinical observation of perceptual instability following ocular paralysis. Following the concept of corollary discharge, the theory of efference copy had the advantage of simultaneously solving three problems: the stability of the visual world during the saccade, the same visual stability across saccades, and the visual constancy problem of allowing the subject to know where an object in space is.