Pattern recognition

Pattern recognition involves making sense of and identifying the objects we see. This topic is closely related to perception, which explains how the sensory inputs we receive are made meaningful.

Two explanations for how we perceive objects are the template matching hypothesis and the feature detection model.

A template is a pattern used to produce items of the same proportions. The template-matching hypothesis suggests that incoming stimuli are compared with templates in the long term memory. If there is a match, the stimulus is identified.

For example the letter A may appear in many forms:

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Either all possible forms have their own template or, with a little 'tweaking', all of the patterns can match one template for the letter A. However, sometimes patterns are ambiguous and fit a template for another class of patterns:

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Feature detection models, such as the Pandemonium system for classifying letters (Selfridge, 1959), suggest that the stimuli are broken down into their component parts for identification. For example, an E has three horizontal lines and one vertical line.

This explanation is less rigid than the template model and allows for a degree of variation in the stimuli. However, this greater flexibility means that the stimuli must be limited to letters. If this were not the case, we would identify any pattern with three horizontal and one vertical lines as an E.

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Evidence from Hubel & Wiesel provides biological support for the feature detection model. Cells in the part of the cerebral cortex devoted to visual information are sensitive to a range of lines with differing orientations. These cells have been called feature detectors.

Feature detectors were found by using microelectrodes to measure the activity of neurons in the brains of anaesthetised monkeys and cats whilst bar shaped visual stimuli were being presented.

Hubel & Wiesel identified three different types of cells. Can you name them?

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Feature detectors provide good evidence for pattern recognition having a biological basis. These experimental techniques cannot be used in humans so the results are generalised. However, the fact that humans are more likely to confuse letters that have similar features (for example, 'b' and 'd' rather than 'b' and 'z') suggests that feature detectors may be at work.

Biological explanations of pattern recognition do not account for the effect of context on perception. In other words, our own expectations and knowledge can influence our perception of patterns. This has been called top-down processing (whereas perception based on features of the stimuli is a bottom-up process).

Evidence that context influences perception comes from Palmer (1975). He presented participants with pictures of familiar scenes and then briefly flashed a picture of an object. If the object fitted with the context of the scene, identification was very good. However, participants performed poorly if the object was not related to the scene.

Scene: Object: Identification?
Bedroom Alarm clock Very good
Bathroom Spade Very bad

This suggests that people's expectations influence their ability to perceive objects.

The evidence suggests that the process of pattern recognition involves elements of both bottom-up and top-down processes. The two processes combine to help us to perceive stimuli quickly and to select what is most likely if patterns are ambiguous.

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