Experimenter naiveté and imaginal judgments

Finke and Kurtzman report that fields of resolution increase with increases in the diameter of both perceived and imagined circular patterns. Their study apparently was conducted by an experimenter who was aware of the hypotheses under test. In contrast to these results, we find no such increase for imagined circular patterns when the experimenter is not aware of the experimental predictions, even though our subjects received imagery training as extensive as that used by Finke and Kurtzman. Our results cannot readily be attributed to differences between the experiments, other than those associated with experimenter naiveté, for we replicated Finke and Kurtzman's observations that (a) the fields of resolution in perception increase with both the diameter and the relative brightness of the patterns and (b) the fields of resolution in imagery do not increase with the relative brightness of the patterns. Our results also challenge Finke and Kurtzman's contentions that imagery and perception are treated as functionally equivalent at area-processing levels of the visual system but not at contrast-processing levels.     

On the relationship between visual imagery and visual perception: Evidence from priming studies

Two priming experiments using a perceptual identification task were conducted to explore the functional and representational overlap between visual imagery and visual perception. The first experiment included a study phase in which primes were either perceived or imaged objects and a perceptual identification test task in which targets were parts of studied and non-studied objects. Imagery primed identification when subjects were instructed to count the parts of the imaged objects in the study phase but not when they were instructed to focus on the global shape of the imaged objects. This result suggests that imagery involves perceptual representations identical to those involved in perception. It also suggests that the representational overlap between imagery and perception depends on the type of images generated, as some images may consist in global shapes only, whereas others may consist in detailed, multi-part shapes. In the second experiment, we used whole objects as target stimuli which provided subjects with more information to identify masked targets and thereby reduced top-down processing. Priming from imagery fell beneath significance, suggesting that this sort of priming is elicited in a large part by a transfer of top-down processes from study to test.     

The effect of target specification on objects fixated during visual search

When a person searches for a target in a cluttered visual field his eye fixations typically fall on objects. The effect of target specification on the probability of fixating different classes of objects was studied. For fields containing objects differing Widely in size, color, and shape: a high proportion of searchers’ fixations were on objects of a specified color, a moderate proportion of their fixations were on objects of a specified size, and a slight proportion of their fixations were on objects of a specified shape. When two or more target characteristics were specified, fixations were generally based on a single characteristic. It is proposed that the specification of a target creates a perceptual structure which the searcher explores. The study of visual fixations, in effect, is the study of the perceptual structure.     

The effect of target specification on objects fixated during visual search

When a person searches for a target in a cluttered visual field his eye fixations typically fall on objects. The effect of target specification on the probability of fixating different classes of objects was studied. For fields containing objects differing widely in size, color, and shape: a high proportion of searchers’ fixations were on objects of a specified color, a moderate proportion of their fixations were on objects of a specified size, and a s light proportion of their fixations were on objects of a specified shape. When two or more target characteristics were specified, fixations were generally based on a single characteristic. It is proposed that the specification of a target creates a perceptual structure which the searcher explores. The study of visual fixations, in effect, is the study of the perceptual structure.     

Comparison processes on visual mental images

Three experiments were conducted to test whether visual mental images and visual perceptual representations possess equivalent structural properties and undergo functionally equivalent comparison processes. In Experiment 1, subjects were required to perform asame-different letter classification in which the two letters were shown in succession. The first letter in the pair either was actually presented (perceptual condition) or had to be actively generated (imaginal condition). Both conditions showed that (1) response latencies fordifferent decisions decreased as a function of the degree of difference (segment effect), and (2) same decisions were faster than the fastestdifferent ones (fast same effect). In Experiment 2, the imaginal condition only was employed, but no imagery instructions were given and very strict time constraints were assigned. In spite of these restrictions, subjects apparently generated and used visual images, as attested by the fact that the results were comparable to those of the previous experiment. In Experiment 3, three experimental manipulations were introduced to prevent the use of visual images. Such manipulations proved effective, as shown by the disappearance of the segment effect. It was concluded that in the visual modality images and percepts are equivalent in structure and are processed in a very similar way.     

Perceptual units in the acquisition of visual categories

A series of experiments provided evidence that the representational structure of categories comprising dot patterns is based on pattern parts and pattern configuration rather than on pattern elements. We found that similarity judgments and postacquisition classification data could not be explained in terms of element-level perceptual units, even for categories of dot patterns with seven of their eight dots in the exact same relative location. The importance of higher order perceptual units was indicated by evidence that the long-term retention of information specific to previously learned category exemplars, which is typical of natural objects, can also be obtained for artificial dot patterns, providing their structure reflects the perceptual characteristics identified in Tversky and Hemenway's (1984) study of natural objects: Members of the same category had to be perceptually distinctive at the level of pattern configuration and perceptually similar at the level of pattern parts. The level of within-category similarity for a set of categories (relative to between-categories similarity) did not predict whether item-specific information would be retained; long-term retention appears to require both within-category similarity and dissimilarity, but at different levels of perceptual structure.     

Reduced visual acuity is mirrored in low vision imagery

Research has examined the nature of visual imagery in normally sighted and blind subjects, but not in those with low vision. Findings with normally sighted subjects suggest that imagery involves primary visual areas of the brain. Since the plasticity of visual cortex appears to be limited in adulthood, we might expect imagery of those with adult-onset low vision to be relatively unaffected by these losses. But if visual imagery is based on recent and current experience, we would expect images of those with low vision to share some properties of impaired visual perception. We examined key parameters of mental images reported by normally sighted subjects, compared to those with early- and late-onset low vision, and with a group of subjects with restricted visual fields using an imagery questionnaire. We found evidence that those with reduced visual acuity report the imagery distances of objects to be closer than those with normal acuity and also depict objects in imagery with lower resolution than those with normal visual acuity. We also found that all low vision groups, like the normally sighted, image objects at a substantially greater distance than when asked to place them at a distance that ‘just fits’ their imagery field (overflow distance). All low vision groups, like the normally sighted, showed evidence of a limited visual field for imagery, but our group with restricted visual fields did not differ from the other groups in this respect. We conclude that imagery of those with low vision is similar to that of those with normal vision in being dependent on the size of objects or features being imaged, but that it also reflects their reduced visual acuity. We found no evidence for a dependence on imagery of age of onset or number of years of vision impairment.     

Visual perception and visual mental imagery of emotional faces generate similar expression aftereffects

What is the relationship between visual perception and visual mental imagery of emotional faces? We investigated this question using a within-emotion perceptual adaptation paradigm in which adaptation to a strong version of an expression was paired with a test face displaying a weak version of the same emotion category. We predicted that within-emotion adaptation to perception and imagery of expressions would generate similar aftereffects, biasing perception of weak emotional test faces toward a more neutral value. Our findings confirmed this prediction. Adaptation to mental images yielded aftereffects that inhibited emotion recognition of test expressions, as participants were less accurate at recognising these stimuli compared to baseline. While the same inhibitory effect was observed when expressions were visually perceived, the size of the aftereffects was greater for perception than imagery. These findings suggest the existence of expression-selective neural mechanisms that subserve both visual perception and visual mental imagery of emotional faces.     

Symbolic magnitude modulates perceptual strength in binocular rivalry

Basic aspects of magnitude (such as luminance contrast) are directly represented by sensory representations in early visual areas. However, it is unclear how symbolic magnitudes (such as Arabic numerals) are represented in the brain. Here we show that symbolic magnitude affects binocular rivalry: perceptual dominance of numbers and objects of known size increases with their magnitude. Importantly, variations in symbolic magnitude acted like variations in luminance contrast: we found that an increase in numerical magnitude of adding one lead to an equivalent increase in perceptual dominance as a contrast increment of 0.32%. Our results support the claim that magnitude is extracted automatically, since the increase in perceptual dominance came about in the absence of a magnitude-related task. Our findings show that symbolic, acculturated knowledge about magnitude interacts with visual perception and affects perception in a manner similar to lower-level aspects of magnitude such as luminance contrast.     

Visual imagery and knowledge about the visual appearance of objects in patients with posterior cerebral artery lesions

The purpose of this study was to explore whether patients with left posterior cerebral artery (PCA) lesions have defective mental visual imagery and to differentiate whether such a deficit stems from a loss of knowledge about the visual appearance of objects or from an inability to create mental visual images out of preserved visual knowledge. Normal controls and patients with either left or right PCA lesions were asked to verify low- and high-imagery sentences and then to verify pictorial representations of the predicates of high-imagery sentences. High-imagery questions concerned either the shape or the color of objects. In addition, perceptual discrimination of shape and color was assessed. Patients with right PCA lesions were impaired on the perceptual discrimination tasks. Left PCA patients did not significantly differ from controls on low-imagery sentences but scored significantly lower on shape and color questions. Their impairment was distinctly more severe with the pictorial than with the verbal versions of shape and color questions. In comparison to patients with occipital left PCA lesions, patients with temporo-occipital left PCA lesions were selectively impaired on verbal and visual color questions. It is concluded that patients with left PCA lesions lack knowledge about shape and color of objects, but that their ability to convert visual knowledge into mental images is preserved.     

The Relationship Between Visual Illusion and Aesthetic Preference – an Attempt to Unify Experimental Phenomenology and Empirical Aesthetics

Experimental phenomenology has demonstrated that perception is much richer than stimulus. As is seen in color perception, one and the same stimulus provides more than several modes of appearance or perceptual dimensions. Similarly, there are various perceptual dimensions in form perception. Even a simple geometrical figure inducing visual illusion gives not only perceptual impressions of size, shape, slant, depth, and orientation, but also affective or aesthetic impressions. The present study reviews our experimental phenomenological work on visual illusion and experimental aesthetics, and examines how aesthetic preference is influenced by stimulus factors determining visual illusions including anomalous surface and transparency as well as geometrical illusion. Along with line figures producing geometrical illusions, illusory surface figures inducing neon color spreading and transparency effects were used as test patterns. Participants made both of psychophysical judgments and of aesthetic judgments for the same test pattern. Both of geometrical illusions and aesthetic preferences were found to change similarly as a function of stimulus variables such as the number of filling lines and the size ratio of the inner and outer figural components. Also, following specific stimulus variables such as lightness contrast ratio and spatial interval between inducing figural elements (so called ``packmen''), strong effects of color spreading and transparency were accompanied with strong preferences. It seems that the paradigm to investigate aesthetic phenomena along with perceptual dimensions is useful to bridge the gap between experimental phenomenology and experimental aesthetics.     

Visual mental imagery and visual perception: Structural equivalence revealed by scanning processes

The research reported in the present article investigates whether information is represented the same way in both visual mental imagery and the early phases of visual perception. In Experiment 1, the same participants scanned over patterns of dots in a mental image (with images based on a just-seen pattern), during perception, and in an iconic image. The time to scan increasing distances increased at comparable rates in the three tasks. However, in Experiment 2, when mental images were created from information stored in long-term memory, participants scanned more slowly in the mental image condition. Nevertheless, the rates of scanning in the perceptual tasks were highly correlated with the rates of scanning in the imagery tasks in both experiments. The results provide evidence that mental images and perceived stimuli are represented similarly and can be processed in the same way.     

Dissociable effects of irrelevant context on 2D and 3D grasping

Grasping movements directed toward real objects are typically unaffected by irrelevant aspects of the object and its surroundings, indicating that such interactions are based on analytic processing of object shape and size. However, recent findings show that grasping directed toward two-dimensional (2D) objects is subjected to perceptually mediated effects of relative shape and size. It is unclear however, whether context-dependent processing—a hallmark of visual perception—affects 2D grasping in the same fashion. Here, we explored this possibility by comparing the influence of a newly discovered contextual effect on 2D and on 3D grasping. According to the range of standard effect (RSE), the perceptual resolution for a stimulus depends on the range of the other stimuli presented within the same session, with higher resolution obtained under narrow compared to wide context range. In two experiments, participants were asked to grasp 3D and 2D objects embedded in a wide or a narrow range. The results showed that, unlike 3D grasping, which was immune to contextual information, the resolution during 2D grasping was significantly modulated by the range of the irrelevant context. The findings suggest that visuomotor control directed to 2D objects is intruded by irrelevant perceptual information, making it context-dependent     

The hare and the snail: Dissociating visual orienting from conscious perception

A method for investigating attentional effects of peripheral visual objects, independently of perceptual identification, is described. We report an experiment using this method which shows that visual processing of peripheral objects differs radically, depending on whether participants move attention in response to an object or consciously perceive that object. When luminance contrast was reduced, conscious perceptual discrimination of peripheral letters was massively slower and less accurate—but both low and high contrast letters elicited rapid attentional orienting effects and these rapid orienting effects were equal in magnitude across low and high contrast. This pattern is consistent with known differences in luminance sensitivity between the dorsal and ventral visual processing streams, and with rapid dorsal–ventral interaction mediated via re-entrant feedback. Our findings show that the control system responsible for rapid movements of attention is exquisitely sensitive to visual form information at low levels of contrast, and involves a different neurocognitive pathway to that which gives rise to conscious perception.     

Visual discovery in mind and on paper

The effectiveness of mental imagery with and without drawing support (perceptual assistance) in the visual synthesis of novel patterns was studied in three experiments. When the task was to create one recognizable pattern from three simple shapes in a 2-min assembly period, subjects were as likely to produce a recognizable or creative pattern per trial whether mental imagery was augmented by external drawing support or not. When the task was to create as many patterns as possible in a 3-min assembly period, more patterns were produced per trial with external drawing support than without; however, neither the recognizability nor the creativity of the patterns differed. Differences in performance in the visual synthesis task with and without external drawing support are interpreted in terms of limited cognitive resources.     

Top-down control of visual perception: attention in natural vision

Top-down perceptual influences can bias (or pre-empt) perception. In natural scenes, the receptive fields of neurons in the inferior temporal visual cortex (IT) shrink to become close to the size of objects. This facilitates the read-out of information from the ventral visual system, because the information is primarily about the object at the fovea. Top-down attentional influences are much less evident in natural scenes than when objects are shown against blank backgrounds, though are still present. It is suggested that the reduced receptive-field size in natural scenes, and the effects of top-down attention contribute to change blindness. The receptive fields of IT neurons in complex scenes, though including the fovea, are frequently asymmetric around the fovea, and it is proposed that this is the solution the IT uses to represent multiple objects and their relative spatial positions in a scene. Networks that implement probabilistic decision-making are described, and it is suggested that, when in perceptual systems they take decisions (or 'test hypotheses'), they influence lower-level networks to bias visual perception. Finally, it is shown that similar processes extend to systems involved in the processing of emotion-provoking sensory stimuli, in that word-level cognitive states provide top-down biasing that reaches as far down as the orbitofrontal cortex, where, at the first stage of affective representations, olfactory, taste, flavour, and touch processing is biased (or pre-empted) in humans.     

Image generation and processing of generated images in the cerebral hemispheres

Recent computational models describing the contribution of the cerebral hemispheres to visual imagery have suggested an exclusive capacity of the left hemisphere to generate multipart images. A brief review of relevant findings indicates that the evidence presented in support of this suggestion is not entirely compelling; this prompted a reexamination of this issue in a lateral tachistoscopic study on normal adults. Sixteen subjects participated in two experiments in which they had to decide whether or not a lowercase letter contained a segment extending above or below the main body of the letter. This decision was made directly on lowercase letters in one experiment (perceptual task) and on their generated images in the other experiment (imagery task). The quality of the letters (clear or blurred) and the retinal eccentricity of stimulus presentation (small or large) were orthogonally manipulated. The perceptual task yielded no main effect of visual field but a significant interaction of visual field and letter quality. By contrast, the imagery task resulted in a left visual-field superiority but no interaction involving the visual fields--a departure from predictions based on current models of visual imagery. In addition, the pattern of results in the imagery task corresponded to that obtained with blurred letters in the perceptual task, suggesting limitations in spatial resolution of visual images. Implications of these results for models of cerebral lateralization and visual imagery are discussed.     

Spatial versus object visualizers: A new characterization of visual cognitive style

The visual system processes object properties (such as shape and color) and spatial properties (such as location and spatial relations) in distinct systems, and neuropsychological evidence reveals that mental imagery respects this distinction. The findings reported in this article demonstrate that verbalizers typically perform at an intermediate level on imagery tasks, whereas visualizers can be divided into two groups. Specifically, scores on spatial and object imagery tasks, along with a visualizer-verbalizer cognitive style questionnaire, identified a group of visualizers who scored poorly on spatial imagery tasks but excelled on object imagery tasks. In contrast, a second group of visualizers scored high on spatial imagery tasks but poorly on object imagery tasks. The results also indicate that object visualizers encode and process images holistically, as a single perceptual unit, whereas spatial visualizers generate and process images analytically, part by part. In addition, we found that scientists and engineers excel in spatial imagery and prefer spatial strategies, whereas visual artists excel in object imagery and prefer object-based strategies.     

Eye scanpaths during visual imagery reenact those of perception of the same visual scene

Eye movements during mental imagery are not epiphenomenal but assist the process of image generation. Commands to the eyes for each fixation are stored along with the visual representation and are used as spatial index in a motor‐based coordinate system for the proper arrangement of parts of an image. In two experiments, subjects viewed an irregular checkerboard or color pictures of fish and were subsequently asked to form mental images of these stimuli while keeping their eyes open. During the perceptual phase, a group of subjects was requested to maintain fixation onto the screen's center, whereas another group was free to inspect the stimuli. During the imagery phase, all of these subjects were free to move their eyes. A third group of subjects (in Experiment 2) was free to explore the pattern but was requested to maintain central fixation during imagery. For subjects free to explore the pattern, the percentage of time spent fixating a specific location during perception was highly correlated with the time spent on the same (empty) locations during imagery. The order of scanning of these locations during imagery was correlated to the original order during perception. The strength of relatedness of these scanpaths and the vividness of each image predicted performance accuracy. Subjects who fixed their gaze centrally during perception did the same spontaneously during imagery. Subjects free to explore during perception, but maintaining central fixation during imagery, showed decreased ability to recall the pattern. We conclude that the eye scanpaths during visual imagery reenact those of perception of the same visual scene and that they play a functional role.     

What you see is what you get: functional equivalence of a perceptually filled-in surface and a physically presented stimulus

A perceptually filled-in surface, such as occurs during sustained attention to a peripheral stimulus (Troxler fading), can be functionally equivalent to a physically presented stimulus. Observers failed to detect probes that were presented in the location of a filled-in surface that had the same surface attributes as the probes; this was true even though, physically, the probes contrasted with the background. Probe stimuli with surface characteristics different from those of the filled-in surface were detected more often, though not quite as often as when there was no filled-in surface. Together, these findings support the idea that there are two components in perceptual filling: a neural filling-in component and a sustained-attention component, which actively suppresses perceptual processing at the filled-in location. More broadly, they illustrate the interplay of basic visual mechanisms in the creation and representation of visual surfaces and in the coding and detection of changes to these surfaces.