Discuss low-level and high-level explanations of illusory contours Essay Example

remain unaffected (Eysenck & Keane, 2000).

To examine the current evidence regarding the development of illusory contours, it is crucial to differentiate between two primary theories of perceptual organization. According to the bottom-up processing theory, our understanding of the environment around us is primarily influenced by the stimuli received by our sensory receptors. This perspective stems from Gibson's (1979) direct perception theory, which suggests that perception involves gathering extensive information from the visual system and entails minimal or no unconscious cognitive processes or internally generated representations. Gibson also emphasized that the amount of visual data present on retinal images is often underestimated.

Although Gibson's theory is considered a valuable explanation for animal perception in visually guided behavior, its effectiveness in difficult conditions where conceptual representation of the environment is challenging is questionable. Additionally, the theory falls short in offering explanations for various factors like constancies, illusions, and cataract patient experiences (Gross, 1998).

Gregory's theory of cues and hypotheses, also known as concept driven processing or a constructionist approach, presents an opposing view. According to the top-down processing, perception is the outcome of a process that starts with sensory stimulation but also involves making deductions based on previous knowledge and expectations of the world. Therefore, our perception is indirect. The inaccuracy of perception, such as visual illusions, supports Gregory's perspective.

In arguing that visual illusions can result in perceptions that are not physically present in the stimulus, the author suggests that the brain utilizes prior experience to fill in gaps. However, this viewpoint fails to account for the overall accuracy of vision in new situations and variations in adjustment times based on image type (Gross, 1998). When considering

illusory contours, three critical properties include clarity, brightness, and depth (Lesher, 1995); whereas it is not necessary for illusory figures to exhibit all three properties.

An instance of illusory contours emerging without a corresponding illusory figure occurs in offset-grating stimuli. Two primary varieties of illusory contours can be identified: edge-induced and line-induced. Edge-induced illusory contours comprise of cohesive inducing components that contain edges or breaks locally consistent with the background occluding figure with the same brightness. In contrast, line-induced illusory contours represent the ultimate form of edge-induced shapes where the inducers are usually 'thin.' Consequently, the associated illusory contours are not parallel to the inducers, but they are instead approximately perpendicular to them. The Kanizsa triangle features both types of inducers.

According to Lesher (1995), the three circular "pac-men" act as edge inducers, while the thin lines function as line-end inducers. Cognitive theories of illusory contour perception have been proposed by various researchers, with Gregory's (1972) and Rock's (Rock & Anson 1979) being the most notable. These theories suggest that illusory contour formation results from a cognitive-like process of postulation. Illusory contours are seen as solutions to a perceptual problem - "What is the most probable organization that accounts for the stimulus?" While cognitive influences have been found to play a significant role in illusory contours, current research indicates the importance of relatively low-level processes in their formation. Early neural mechanisms for illusory contour completion are suggested by two lines of evidence: (1) neurophysiological data and (2) psychophysical studies of the similarities between real and illusory contours.

von der Heydt and colleagues (von der Heydt et al.1984) documented in a significant study that area V2 of the macaque

monkey contained neural correlates of illusory contours. They discovered that nearly half of the examined cells responded substantially to both drifting bars or edges and the illusory contour produced by drifting line gratings. However, these cells were not merely responding to individual line-ends since, despite not reacting to gratings with only 2 or 3 bars, a typical cell's responsiveness grew stronger as additional bars were introduced until reaching a saturated level of activity.

According to Von der Heydt et al. (1984), they conducted a study on neural responses to notch stimuli. These stimuli consisted of dark rectangles with missing parts, which created an illusion of a rectangle. The study found that cellular activity decreased as the separation between notches increased, and was significantly reduced when only one notch was present. This decline in neural activity correlated with the disappearance of the illusory figure perceived by the subjects. The results suggested that the cellular responses to variations in illusory contours were similar to human psychophysical responses to the same variations. Although some have referred to these discoveries as "illusory contour cells," Von der Heydt et al. presented their findings as such.

In 1984, researchers attempted to differentiate between the relationship of stimulus-response and perceived entities, using terms like "illusory contour stimuli" and "anomalous contours" borrowed from Kanizsa. Psychophysical studies also provide evidence of both real and illusory contours being processed similarly in the early stages of visual processing, as shown in similarities in motion aftereffects, tilt aftereffects, orientation discrimination, and orientation masking demonstrated by Smith and Over in 1975 and 1979. Of these, tilt aftereffects are especially noteworthy.

Looking at lines that are oriented counterclockwise from

the vertical for several seconds will result in a tilt aftereffect. When one is then introduced to a test stimulus of vertical lines, these lines will seem to be tilted clockwise, moving away from the adapting orientation. Recent studies have provided convincing evidence that tilt aftereffects can cross over between illusory and real contours. Paradiso et al. (1989) found that adapting with actual lines can impact the perception of illusory contour orientation, and vice versa.

There is a significant inquiry about the similarity in status between real and illusory contours. The attribution of motion and tilt aftereffects to short term habituation in early visual stages has led to the belief that real and illusory contours have shared internal processes at an early level of the visual system, according to psychophysics research (Movshon et al. 1972). Evidence suggests the functional equivalence of real and illusory contours in the operation of the visual system (Lesher 1995). Paradiso et al. also support this notion.

In 1989, a study was conducted to compare the tilt aftereffects caused by adaptation to illusory contours and real lines. The study found that both types of stimuli produced strong tilt aftereffects during adaptation and testing phases. It was also discovered that adaptation to either type of stimulus could induce an aftereffect when tested with the other type. Paradiso and colleagues confirmed this finding, showing that adaptation to real lines resulted in a strong aftereffect when tested with illusory contours, whereas adaptation to illusory contours produced a weaker aftereffect when tested with real lines.

The presence of a tilt aftereffect in response to illusory contours indicates that there are orientation-selective neurons that react to

such stimuli. This asymmetry is attributed by the authors to the uneven distribution of receptive field types across areas V1 and V2, with illusory contour-responsive cells being primarily found in V2. This suggests that real and illusory contours share an early visual pathway. Additionally, Grosof and colleagues (1993) proposed the existence of neurons in macaque's V1 region that respond to line-end stimuli similar to offset gratings in a recent study.

The role of neurons in illusory contour perception is still debated and requires additional testing (Lesher, 1995). Nevertheless, it appears that the early stages of perception are where illusory contours are represented. While connecting single cell activity to perceptual occurrences can be challenging, the data imply that neural completion of a presence, rather than "ignoring an absence," contributes to perceptual boundary completion.