SENSORY PERCEPTION

Sensory perception



Sensory perception

Introduction

Both sensation and perception are functions of the brain—specifically, a part of the brain called the cerebral cortex, or cerebrum (see figure below). Nerve impulses from sensory receptors are transmitted to particular parts of the brain. The brain interprets them as sensations, an awareness and localization of a stimulus. A stimulus is a change in the internal or external environment that leads to a response. A perception involves giving meaning to a sensation based on what we have experienced and learned. For instance, stepping on a tack will cause a sensation of pain, whereas an awareness of being injured would be considered a perception. Perception is important in determining how we will respond to a particular stimulus.

Role of Sensory Receptors

Sensations, and the perceptions they evoke, begin with sensory reception, the detection of a stimulus (or, more accurately, the energy of a stimulus) by sensory receptors. Sensory receptors are anatomical structures made up of special cells that respond to specific changes in their environment (stimuli). In so doing, they provide the central nervous system (brain and spinal cord) with information about conditions both inside and outside the body. In this way, sensory receptors are the physical links between your central nervous system and the environment.

Sensory receptors work by first detecting a stimulus and then translating that energy into electrical signals, which are conducted by nerve cells, or neurons, to the central nervous system. Turning a stimulus into an electrical signal is called transduction (a transducer is a device that changes one form of energy into another). Specialized sensory receptors are needed to detect stimuli because the brain has no such ability. That is, the brain is designed to receive electrical signals from the nervous system; beyond that, it is essentially "blind" to all other forms of stimuli. This explains why neurosurgeons may perform procedures on the brain of an awake patient. Only a local anesthetic is needed for the scalp because the brain cannot directly sense touch or pain.

The initial response of a sensory receptor to a stimulus is to change its cell membrane permeability to ions—how many and what kinds of ions it lets through. In other words, in response to an appropriate stimulus, transport pathways for specific ions open and/or close, depending on the type of stimulus and receptor.

Vision

Every eye has a blind spot where the optic nerve enters the eye and where photoreceptors are absent. We normally do not notice this because the brain fills in the missing information. The brain can give us the illusion of an uninterrupted visual field, but it cannot change the size of the blind spot because the area where photoreceptors are absent is a fixed anatomical feature. Diseases of the retina can cause photoreceptors to malfunction, which may cause an apparent enlargement of the anatomical blind spot or the appearance of additional blind spots at other locations.

Brain function testing by blind spot mapping has been reported in only one published ...