Sensation and Perception Lecture: 5 Sensation and Perception

Sensation: Receiving Messages about the World We are aware of the outside world and the internal world of our own bodies only because we have a number of sense organs able to receive messages. These organs enable us to see, taste, hear and experience such feelings as happiness, pain etc. Sense organs operate through sensory receptor cells, which receive outside forms of energy(light , vibrations, heat) and translate them into neural impulses that can be transmitted to the brain for interpretation.

Contd. Sensation: The process of receiving , translating and transmitting messages from the outside world to the brain. (here outside is used broadly) Stimulus: The term stimulus comes from the action of stimulating sensory receptor cells. Virtually anything that can excite receptor cells can be stimulus. For instance food becomes a stimulus through your senses of taste , smell and sight. When you eat too much at a dinner party , the bloated stretching of your stomach is a very noticeable one.

Transduction: Translating messages for the Brain Energy from the stimuli can not directly go to the brain as light , sound and other forms of energy from the outside world can not travel through the nerves and the brain can not understand what they mean. To be understandable for the brain , sensory messages must be translated into neural impulses that the neurons carry and the brain understands. Thus the translation of one form of energy into another is called “Transduction.”

Process of Transducing Sense organs transduce sensory energy into neural energy. At the centre of each sense organ are receptor cells that do the transducing. These receptor cells are specialized neurons which are excited by specific kind of sensory energy and give off neural impulses from their axons and so on. In every case the receptor cells give off coded neural impulses that carry the transduced sensory message to one of the sensory areas of the brain .

Radio Waves - Example We can only be aware of a stimulus if we have receptor cells that can transduce it. For instance we can not see a radio wave although it is as real as light reflected to our eyes from an apple for example, but since we do not have receptor cells that can transduce this energy so we can not see it. We only know that radio waves exist because radios physically transduce them into sound waves which are in turn tranduced into by our ears into neural messages to the brain. There are many forms of energy in the world that we are not aware of because we do not have receptor cells that can transduce them .

Sensory Limits Even when the receptor cells are there that can transduce a kind of sensory message , not every message will be strong enough to be detected. The term “Threshold ” refers to the lower limits of sensory experience. The two primary kinds of thresholds are: the smallest magnitude of a stimulus that can be detected (b) the smallest difference between two stimuli can be detected.

(contd.) Absolute Threshold: The Absolute threshold is the smallest magnitude of a stimulus that can be detected by an organism.

Difference Threshold: The smallest difference between two stimuli that can be detected half the time is called the difference threshold. Just Noticeable Difference (JND)

Sensory Adaptation (Contd.) Since the individual’s sensitivity to a stimulus varies from time to time, there can be many reasons for that such as fatigue or inattention but sensory adaptation is one of the major causes. When a stimulus is continuously present or repeated at short intervals, the sensation that the same amount of sensory energy causes becomes gradually weaker . In part because the receptor cells become fatigued. It happens to some extent in all the senses; loud sounds and offensive odors , fortunately also seem less intense ad time goes by.

Psychophysics The specialty area within the field of Psychology that studies sensory limits , sensory adaptation, and related topics is called “Psychophysics”. The subject matter of this field is the relation between the physical properties of stimuli and the psychological sensation they produce.

Vision: Sensing Light The eye is much like human camera. The lens focuses a visual image on the retina of the eye, which contains two kinds of sensory receptor cells, the rods and the cones. These transduce the wavelength, amplitude and complexity of the light waves into neural messages, which are coded by the retina before travelling along the optic nerve to the brain. The two kinds of receptor cells perform their jobs somewhat differently . Cones work best in intense light, provide good visual acuity and transduce information about colour. Rods work best in weak light , do not provide good acuity , and do not code information about colour.

Contd. The eye does not function well when the intensity of light suddenly changes , but it quickly regains its sensitivity through the process of light and dark adaptation. Light adaptation: Regaining sensitivity of the eye to bright light following an increase in overall illumination. Dark Adaptation: Increased sensitivity of the eye in semi darkness following a reduction in overall illumination.

Contd. There are two major theoretical explanations of how the visual system transduces color. One states that three different kinds of cones are most sensitive to the light of different wavelengths. The other suggests that two kinds of color processing mechanisms in the visual system process complimentary colors. Each theory is correct at different stages of the information processing about the wavelength of light.