Sensory, Attentional and Perceptual Processes

Sense organs have functional limitations because they can respond only to stimuli of an optimal intensity. Very dim or very bright light may not be seen, and very faint or very loud sounds may not be heard. The same is true for other sense organs too.

To be noticed, a stimulus must cross a minimum level called the absolute threshold (absolute limen, AL). Also, to notice that two stimuli are different, there must be a minimum difference between them, called the difference threshold (difference limen, DL).

Sensory processes do not depend only on the stimulus itself. The sense organs, their neural pathways, and the brain areas that receive the impulses are also important. Any structural or functional defect in any of these may lead to partial or complete loss of sensation.

Attention is the process through which certain stimuli are selected from a group of others. It helps us notice and register only some of the many stimuli that reach our sense organs.

Its main properties are:
- Alertness: readiness to deal with incoming stimuli.
- Concentration: focusing awareness on some objects while excluding others for the moment.
- Search: looking for a specified subset of objects among many.
- Effort allocation: attention requires effort.
- Focus and fringe: the focus is the center of awareness, while the fringe contains vaguely noticed objects or events.

The determinants of selective attention are of two kinds:

### External factors
These are related to the features of stimuli:
- Size
- Intensity
- Motion
- Novelty
- Moderate complexity
- Human photographs are attended more than inanimate objects
- Rhythmic auditory stimuli are attended more than verbal narrations
- Sudden and intense stimuli easily draw attention

### Internal factors
These lie within the individual:
- Motivational factors: biological or social needs, such as hunger
- Cognitive factors: interest, attitude, and preparatory set

### Difference between selective and sustained attention
- Selective attention is concerned with the selection of a limited number of stimuli from many stimuli.
- Sustained attention is concerned with concentration and the ability to maintain attention on an object or event for longer durations; it is also called vigilance.

So, selective attention chooses what to attend to, while sustained attention is about keeping attention on one task for a longer time.

The main proposition of the Gestalt psychologists is that we perceive the visual field as an organised whole, not as a collection of separate parts. According to them, the form of an object lies in its whole, and this whole is different from the sum of its parts.

They also stress that perception is guided by a tendency toward a good figure or pragnanz, which means we naturally organise stimuli into meaningful wholes.

Perception of space takes place through our ability to convert the two-dimensional retinal image into a three-dimensional perception. The process of viewing the world in three dimensions is called depth perception or distance perception.

In this process we use cues of depth from two main sources:
- Monocular cues: work with one eye
- Binocular cues: require both eyes

These cues help us judge size, shape, direction, distance, and depth of objects in space.