Lab Practical Special and General Senses

Vision Focal Point Test

Purpose: Measures the ability of the lens to change shape (accommodation), which determines the closest distance an object can be focused sharply.

Result Significance: This distance increases with age due to lens stiffness (presbyopia).

Method: Slowly move a pen toward the eye until it blurs, then measure the distance (cm).

Dominant Eye Test

Purpose: Identifies the eye that the brain prioritizes for precise alignment and spatial input.

Method: Create a small opening with hands, center it on a distant object, and bring it toward the face. The eye the opening aligns with is the dominant one.

Vision: Peripheral Vision

Purpose: Determines the limits of the visual field outside of central focus, demonstrating the difference in visual clarity between the periphery and the center.

Anatomy: Peripheral vision relies heavily on rods (motion/low light) and lacks the sharp focus provided by cones in the central retina.

Method:

1. Move a paper from the side inward: note where the partner can sense the paper versus where they can read the phrase.

2. Move a colored object from behind toward the side until it just enters the visual field.

Vision: Visual Acuity (Cones and Rods)

This experiment investigates how our ability to see details (visual acuity) changes based on light levels, demonstrating the function of the two main photoreceptors: cones and rods.

Purpose

To compare visual acuity under photopic (light) and scotopic (dark) conditions to illustrate the primary roles of cones (detailed, color vision in light) and rods (peripheral, low-light vision).

Method (Based on Snellen Chart)

1. Light Conditions (Cones Active): Stand 20 feet in front of a Snellen Eye Chart (which measures acuity) in a brightly lit room.

• Determine and record the smallest line of letters you can clearly read.

2. Dark Conditions (Rods Active): With the shades drawn and the lights dimmed (or after spending time in the dark), repeat the procedure, standing in the same place.

• Determine and record the smallest line you can read.

3. Observation: Compare the difference in visual acuity between the light and dark conditions. Acuity is expected to be significantly lower in the dark.

Cones (Central Retina)

Provide high visual acuity and color vision but require bright light. They govern vision in the light condition.

Rods (Peripheral Retina)

Provide poor visual acuity and no color vision but are extremely sensitive to light. They govern vision in the dark condition.

Blind Spot Test

This experiment demonstrates the optic disc, a small area on the retina that contains no photoreceptors, resulting in a blind spot in our visual field.

Purpose

To locate the optic disc, which is the point where the optic nerve leaves the eye and blood vessels enter. Since there are no rods or cones here, it creates a small natural blind spot.

Method

1. Preparation: Hold the paper with the cross (+) and the dot (\bullet) at arm's length.

2. Procedure: Close your left eye. Stare continuously at the cross (+) with your right eye.

3. Movement: Gradually move the paper closer to your face while keeping your focus fixed on the cross.

4. Observation: At a specific distance (usually 15-20\text{ cm}), the solid black dot (\bullet) will suddenly disappear from your vision. This is the blind spot.

5. Repeat (Both Eyes Open): Repeat the exercise with both eyes open. The dot will not disappear.

Blind Spot (Optic Disc)

This area contains the nerve fibers leading away from the eye.

Why It Disappears: When the dot's image falls precisely onto the optic disc, there are no light-sensitive cells (photoreceptors) to detect it, so the brain registers nothing.

Why It Doesn't Disappear with Both Eyes Open: In normal binocular vision, the visual field of the left eye covers the blind spot of the right eye (and vice-versa), and the brain fills in the missing information, so you never perceive a hole.

Vision: Astigmatism Test

Investigates focus irregularities, the functions of the photoreceptors (rods/cones), and a blind area in the visual field.

This test uses the Astigmatism Chart (or sunburst chart) to detect astigmatism, a common focusing problem.

• Purpose: To detect if the curvature of the cornea or lens is irregular, causing light to focus unevenly on the retina.

• Mechanism: Astigmatism prevents light from focusing on a single point. Instead, it focuses on two different points, corresponding to different axes (meridians).

• Method:

1. Look at the center of the diagram with one eye at a time.

2. Observe the lines radiating from the center (like spokes on a wheel).

3. If astigmatism is present, the lines along one axis (e.g., 12-6 or 3-9) will appear sharper or darker than the lines on the perpendicular axis.

4. If there is no astigmatism, all lines will appear equally dark.

Hearing: Sound localization

This test determines the ability to locate the source of a sound in space.

Purpose: To demonstrate that the brain locates sound sources using two key factors: the difference in the loudness of the sound reaching each ear, and the time of arrival of the sound at each ear.

• Concept: Sound arriving at the nearer ear is both louder and earlier than at the farther ear. The brain processes these differences.

Method:

1. Partner covers both of their eyes.

2. Hold a pocket watch at an audible distance (about 6 inches) from their ear.

3. Have a partner locate the position (front, back, sides, above/below) of the sound by pointing.

4. Observe which positions make the sound easiest or hardest to locate.

Hearing: Frequency Range

This test estimates the range of sound frequencies a person can perceive.

Purpose: To determine the upper and lower limits of the audible sound frequency spectrum for the subject.

Method:

1. Obtain three tuning forks: one low-frequency (\sim 75\text{ to }100\text{ Hz}), one medium-frequency (\sim 500\text{ Hz}), and one high-frequency (\sim 1000\text{ Hz}).

2. Strike the low-frequency fork and hold it close to the partner's ear.

3. Repeat with the other two forks.

4. Record which frequency was heard most clearly and which was heard least well.

Hearing: Weber Test (Conductive and Sensorineural Deafness

This test helps distinguish between the two main types of hearing loss: conductive and sensorineural.

Purpose: To determine if hearing loss is conductive (problem with sound waves reaching the inner ear, e.g., eardrum damage, earwax) or sensorineural (problem with the inner ear or auditory nerve).

Method:

1. Strike a tuning fork and place the handle on the top of the forehead (medially).

2. Ask the subject if the sound is heard equally in both ears, or if it is louder in one ear (lateralization).

• Interpretation:

• If sound is heard equally, hearing is normal or equally impaired in both ears.

• If sound lateralizes (is louder) to one ear:

• Deafness in the louder ear indicates conductive deafness (sound is heard better by bone than air, overcoming the problem in the outer/middle ear).

• Deafness in the quieter ear indicates sensorineural deafness (the affected inner ear/nerve cannot process the sound).

Hearing: RinneTest (Bone and Air Conduction Hearinf)

This test directly compares the efficiency of sound conduction through the air (normal hearing pathway) versus through the skull bone.

Purpose: To compare bone conduction (sound traveling through the skull to the inner ear) to air conduction (sound traveling through the auditory canal) to diagnose the type of deafness.

Method:

1. Strike the tuning fork and place the handle on the subject's mastoid process (bone behind the ear).

2. When the sound is no longer audible, quickly move the still-vibrating prongs next to the auditory canal (air conduction).

3. If the subject can still hear the fork via air, the result is Positive (normal hearing).

4. If the subject cannot hear the fork via air, the result is Negative (indicates conductive deafness because bone conduction is greater than air conduction).

5. Repeat for the opposite ear.

Equilibrium: Balance Test

This is a simple test of static and dynamic balance.

Purpose: To determine the function of the equilibrium apparatus by observing the ability to maintain balance while walking.

Method:

1. Have your partner walk a straight line, placing one foot directly in front of the other (heel-to-toe).

2. Observe if they can walk without undue wobbling or experience dizziness.

Equilibrium: Barany Test (Induction of Nystagmus and Vertigo)

This experiment evaluates the semicircular canals and should be conducted as a group effort to protect the subject.

Purpose: To test the function of the semicircular canals (the dynamic equilibrium sensors) by inducing a spinning sensation (vertigo) and involuntary eye movements (nystagmus).

Method:

1. Subject sits on a rotating chair with their head inclined forward \sim 30^{\circ} (to stimulate the horizontal semicircular canal). Eyes are open.

2. Rotate the chair approximately 10 revolutions in 10 seconds, then stop abruptly.

3. Immediately observe the direction and duration of the resulting nystagmus (involuntary, jerky eye movement).

• Concept: The sudden stop causes the fluid (endolymph) in the semicircular canals to continue moving, bending the sensory hairs, which the brain interprets as continued rotation (vertigo). This reflexively causes nystagmus.

Equilibrium: Romberg Test

This assesses the integrity of the spinal cord's dorsal white column, which transmits impulses from proprioceptors (sensors in muscles/joints) involved in posture.

Purpose: To assess the part of the spinal cord responsible for transmitting non-visual information about body position needed for balance.

Method:

1. Partner stands erect with their back to a blackboard, eyes open, for \sim 2 minutes. Observe for gross swaying.

2. Repeat the test with the partner's eyes closed.

3. Observation: Record the degree of side-to-side and front-to-back swaying. Increased swaying when eyes are closed suggests a problem with the proprioceptive pathways in the dorsal column.

Equilibrium: Nystagmus

involuntary rolling or trailing of the eyes followed by their rapid movement in the opposite direction.

The eyes slowly move in the direction of the perceived spin (slow phase) and then quickly jump back (rapid phase). The resulting pattern is the nystagmus you observe

Equilibrium: Vertigo

sensation of dizziness and rotational movement even when such movement is not occurring or has ceased

Olfactory: Olfactory Adaptation

This experiment determines the time it takes for the sense of smell to adapt (fatigue) to a constant odor.

Purpose: To demonstrate olfactory adaptation—the sensory fatigue that causes the olfactory receptors to respond less strongly to a constant, repeated scent over time.

Mechanism: This occurs due to receptor fatigue in the olfactory pathway and reduced input processing in the olfactory cortex.

• Method:

1. Hold a container of a single scent (wintergreen oil or isopropyl alcohol) and waft the fumes toward your nose.

2. Start a stopwatch and breathe through your nose to smell the odor.

3. Stop the watch when you no longer sense the odor and record the time.

4. Repeat with a different odor after a break to show that adaptation is specific to the original scent.

Gustation: Taste Mapping

This is the standard, though largely outdated, lab experiment to map the regions of the tongue for different basic tastes.

Purpose: To explore gustation (taste perception) and identify the areas of the tongue designated for the five basic tastes: sweet, salty, bitter, sour, and umami.

• Note: Modern research shows taste receptors for all five categories are spread over the entire tongue, not confined to specific zones.

Anatomy: Taste receptors are found in taste buds on the surface of the tongue and soft palate.

Method:

1. Obtain test solutions for each taste (e.g., sugar for sweet, salt for salty).

2. Use a cotton swab to apply each solution to different, specific regions of the tongue diagrammed in your lab manual (tip, sides, back).

3. Record the region of the tongue where each taste is perceived most strongly.

Gustation: Taste of Phenylthriocarbamide

This genetic experiment determines if an individual is a "taster" or a "non-taster" of the chemical PTC.

• Purpose: To determine individual genetic differences in the ability to taste PTC.

• Concept: Approximately 30% of the population are non-tasters (cannot taste anything), while the rest perceive it as either sweet or bitter.

• Mechanism: The ability to taste PTC is determined by a specific gene for a taste receptor protein.

• Method:

1. Obtain a piece of PTC test paper.

2. Place it in your mouth and chew it.

3. Determine and record whether the paper tastes sweet, bitter, or has no taste.

4. Collect class data to observe the distribution of tasters vs. non-tasters.

Sense of Touch: Two-Point Discrimination Test

This test measures the density of touch receptors on different body areas.

Purpose: To determine the two-point threshold, which is the minimum distance required for a person to feel two distinct touch points simultaneously, rather than just one.

Concept: Areas with the highest density of touch receptors (like fingertips) have the smallest two-point threshold and thus the greatest sensitivity. These sensitive areas also correlate with areas that receive the greatest motor innervation (fine motor control).

Method:

1. Use a caliper to touch the skin simultaneously at two points.

2. Starting with the caliper arms close together, gradually increase the distance between the two points.

3. The smallest distance (in \text{mm}) at which the subject feels two distinct points is the two-point threshold.

Sense of Touch: Tactile Localization

This test measures how accurately the brain can pinpoint the location of a touch.

Purpose: To determine the ability to accurately locate which portion of the skin has been touched, known as tactile localization.

Concept: The ability to localize the touch relates to the size of the corresponding "touch" field in the brain's somatosensory association area (parietal lobe). Areas with better touch receptor representation show a lower error of localization (more accuracy).

Method:

1. The subject's eyes are closed.

2. The experimenter touches the skin with a pointed marker.

3. The subject immediately attempts to touch the exact point with a different colored marker.

4. The error of localization is the measured distance (in \text{mm}) between the two marks.

Sense of Touch: Adaptation of Touch Receptors

This test measures the phenomenon of touch awareness fading over time.

Purpose: To demonstrate adaptation, the process where the intensity of nerve impulses transmitted by sensory receptors declines when a stimulus is applied continuously for a long period. This causes the loss of conscious awareness of the stimulus.

Concept: Touch receptors, especially those for pressure, are rapidly adapting (phasic). This is why you quickly stop feeling your clothes against your skin.

Method (Pressure Adaptation):

1. Place a coin on the subject's forearm while their eyes are closed.

2. Determine the duration of time (in seconds) the subject can feel the pressure sensation.

3. Repeat with a stack of more coins (increased pressure) to see if the duration of sensation changes.