Perception

What is real? How do you define real? If you are talking about what you can touch, see, hear, smell, taste and feel etc, then real is simply electrical signals interpreted by your brain. We all live in a dream world just like in matrix movie which is different from the reality. Our whole perception of the environment of the external world is based on neural signals. Plato’s famous allegory of cave also illustrates that the world we perceive is different from the reality. For instance, In Schizophrenics when they have auditory hallucinations to them, they are real because their primary auditory cortex is active during hallucinations. Their brains have no means to distinguish between the sound inputs from the outside world and hallucinations.

      Perception is the process by which the cognitive system constructs an internal representation of an outside world. Perceptions are built from sensations (sensory inputs e.g. vision, hearing, touch, taste, smell etc. ) and are influenced by available knowledge, expectations, experiences.  In general, perception is the process of interpreting something that we see or hear in mind and use it later to judge and give a verdict on a situation, person, group etc. It involves a sequence of steps: 1)   Selection of stimuli in the environment – this mainly happens unconsciously 2) Organizing information about stimuli 3) Interpretation

Perception is based on internally driven attention (bottom up processing) and influenced by externally driven attention (top down modulation). Bottom up processing begins with stimulus itself. It is unidirectional and each successive stage carries out more complex analysis of the input. Vision as example : Light → Eye → Retina → Rods & Cones (receptors) → neural activity → primary visual cortex (lines and edges) → successively higher visual areas (up to “face regions” and alike)

Top down processing involves use of previous knowledge, expectations and contextual processing in pattern recognition. “Higher level” brain areas influence neural processing in lower level perceptual areas. For example, Understanding difficult handwriting is easier when reading complete sentences than when reading single and isolated words.

 

Key Issues in perception:

1)  It is not an accurate representation of reality. Instead, perception is an interpretation of sensory input. We know from experiences that senses may deceive us and one and the same input may be interpreted in different ways.

Fig a) It can be interpreted as a duck or rabbit                                                                                                            

 Fig b) this dress may appear golden white  to some or bluish black to others or may be different

 2) Sensation Adaptation: We often don’t perceive stimulus that remains constant over prolonged periods of time. While it reduces our awareness of constant stimulus, it helps free up our attention and resources to attend to other stimuli in the environment. Eg. When you enter a cold swimming pool or into a hot tub, the water may feel unpleasantly cold or too hot, but eventually, your body adjusts to the temperature and it feels only mildly cold or mildly hot. 

3) Signal Detection Theory: The ability to identify a stimulus when it is embedded in a distracting background. Eg. 1) You can hear your friend’s conversation even in a loud party. 

4) Inattentional blindness: It is a failure to notice something that is completely visible due to lack of attention. Eg. In a study conducted by Daniel Simons and Christopher Chabris (1999), participants watched a video of people dressed in black and white passing basketballs. Participants were asked to count the number of times the team in white passed the ball. During the video, a person dressed in a black gorilla costume walks among the two teams. You would think that someone would notice the gorilla, right? Nearly half of the people who watched the video didn’t notice the gorilla at all, despite the fact that he was clearly visible for nine seconds. Because participants were so focused on the number of times the white team was passing the ball, they completely tuned out other visual information. 

 Psychophysics is a field which studies relationship between physical stimuli and their perception.   Behaviour/percept is a function of stimulus input i.e.

 ψ = f(ϕ)

ψ = percept/behaviour and ϕ =input stimulus

Three central questions in psychophysics:

1)    Detection threshold (Absolute threshold) – What is the minimum strength of a physical stimulation so that we notice its presence? Example: Graph of absolute auditory threshold for people of different ages is shown. It shows that detection threshold increases with aging.

2)    Relative threshold (just noticeable difference or jnd) – smallest detectable difference between two stimuli. It is given by Weber law.

        ∆I/I=k

I = intensity stimulus 1

∆I = difference in intensity of stimulus 2

k = constant (Weber’s constant)

In approximation, the jnd is proportional to the stimulus intensity. The higher the intensity, the larger the jnd and vice versa.

3)    Scaling – How does the perceived intensity change with a change of the physical stimulus? or How much does the physical stimulus need to change to perceive a doubled intensity?

It is not linear: For instance a light twice as intense does not lead to the subjective perception of a light twice as bright.

By experiments it is found that the Perception of the sensory events of an action is different for our own actions and those of others. Our own actions are felt “less intense”, because we already expect its effects. For example: 1) we (usually) can’t tickle ourselves. 2) Lack of this expectation in schizophrenics may explain why they attribute actions to an outside agent

References : Cognitive Pyschology by Andre Szameitat