Sensation and Perception

Important Questions How can the immaterial human soul interact with the material world? How can the soul sense and acquire knowledge about the outside world?

Modern vs. Medieval Both focused on physical and physiological processes (ex. What the eye is made of) Medieval psychology was mainly focused on the function of the eye With regards to the eye, modern psychology focuses on neural processes that occur after the eye has taken in stimuli and transformed them into neural impulses

What is the human soul? Different views: Aristotelian - the soul is the functioning of the body Platonic - the soul makes active use of the body Implications for different views: Intromission Theory – explains vision by what enters the eye from the environment (Passive Perception) Extramission Theory – spirit flows out from the eyes and interacts with the world (Active Perception)

Medieval Perception Things in the physical world have unique qualities that can been seen or touched, and are called sensibles. (ex. Temperature can be sensed, but magnetism cannot) Sensibles cause impression in the sense organ of the perceiver, which is called the sensible species. Sensible species provides limited information about the qualities of what is being perceived. (ex. Angle of view) Species allows for the determination of the form of an object. The real form of an object was a complex debate at the time.

Aristotle’s View Two kinds of perceptible sensory qualities: Special objects of sense – perceptible to only a single sense (ex. Color is special object of sight, sound of hearing, odor of smell, etc.). Organized into categories of contraries (ex. Heat is contrary to cold, sweet contrary to bitter) Common Sensibles – perceivable by more than one sense (ex. Movement, number, and magnitude). Are not perceived through sense organs, instead through general sensibilities. Sensory error only occurs with common sensibles There are only five senses, and each one has at least one special sensible or object of sense. Each sense requires a medium between the organ and object.

Medieval Understanding of Aristotle Aristotle's approach was very popular in the Middle ages and many doctrines were widely copied While popular, Aristotle was not dogma and further inquiry was required Two contributing factors: Rival theory of vision – involved the extramission of rays of light or spirits Aristotle's incomplete account – physiological processes, and some sense organs were not specified

Physiological Processes Combined Aristotle’s account of perception with Galen’s medical description of animal spirits Animal spirit – fills the sensory nerves that connect the sense organs to the front ventricles of the brain. Conveys sensory information. According to Avicenna, common sense was located in the front ventricles

Bacon’s Account of Vision Franciscan monk who studied vision and Alhazen’s works Bacon’s version of the inner senses: Three ventricles in the brain First one, nearest the forehead, contained the two faculties of common sense and imagination, which judge the special and common sensibles The other two ventricles contained the faculties of estimation, memory, and cogitation, which assessed more cognitive qualities, such as whether something perceived was useful or harmful

Bacon’s Optical Theory Bacon’s account of optics is very similar to Alhazen: Objects radiate light in all directions Light rays that hit the lens of the eye at a perpendicular angle pass through at full strength Rays hitting the eyes at an oblique angle are weaker and are masked by perpendicular rays This idea solves the problem that rays emit from all points of an object and hit all points of the lens Perpendicular rays create a 2D image

Bacon’s Optical Theory Forms are extracted where the species from both eyes mingle in the common nerve To reach the common nerve, rays refract in the vitreous humor Refraction is important because otherwise the image will be formed in the eye itself rather than the common nerve Because the nerve is not a straight line, Bacon has trouble explaining how the rays travel along it and defers to the “processes of the soul”

Theoretical Problems Bacon attempts to address why the species does not mix when transmitting though the medium and stays separate He asserts that the animal spirit is responsible for this process So although he adopts Alhazen’s intromission theory, he does not completely abandon extramission theory

Size Constancy and Illusion Perception is not only sensory processes but also involves reasoning and knowledge Distance is determine by observing other objects around the distant object for reference Knowledge is necessary because the other objects must be known in terms of measurement At extreme distances, judgment becomes more difficult (ex. Sun and moon appear as flat circles) Estimating size requires reasoning skills Size of the angle an object makes is not enough to know the size of the object, it is also necessary to know the distance