Senses and Perception - Lecture Notes

Week 6-7: Senses and Perception

Introduction

• Overview of the lecture topics:
  • Sensory systems, receptor neurons, and sensory organs.
  • Encoding of sensory information.
  • Vision and other sensory modalities.
  • Practical aspects of senses and perception.

Art and Communication

• Visual art and storytelling are inherent reflections of cultural and social norms.
• Art emerges and develops as a behavioral activity in different evolutionary and historical contexts.
• Communication reflects subjective sensations and is a way of prehending and comprehending the world.
• Rock art, the oldest prehistoric art form, dates back to the Upper Palaeolithic period (50,000 to 12,000 years ago).
  • Oldest finds proposed in Australia around 60,000 years ago.
    *Examples of animal architecture and art include:
  • Australian bowerbirds building bowers for courtship and mating.
  • Nest ornaments found in fish, sand gobies, raptors, owls, and stingless bees.

Measuring Sensory Experiences

• Methods to explore and measure sensations:
  • Detection
  • Discrimination
  • Magnitude estimation
  • Matching
• Two-alternative forced choice (2AFC) method:
  • Response is either correct or incorrect (binary variable).
• Magnitude estimation:
  • Response is a value within a defined range (continuous variable).
• Threshold:
  • The difference between stimuli that an observer can discriminate correctly above chance with a predefined level of accuracy (e.g., 75%).
  • Weber’s Law describes discrimination thresholds.
  • To be just noticeably different (JND), any stimulus ($R$) must differ from another ($\Delta R$) by a constant proportion ($k$).
  • $\frac{\Delta R}{R} = k$

Stimulus and Observer Dimensions

• Stimulus dimensions:
  • Intensity and noise
• Observer dimensions:
  • Sensitivity, speed, accuracy, internal representations, adaptation state, and neural noise

Weber's Law

• Weber’s Law describes discrimination thresholds

Noise and Signal Detection

• Noise in stimuli, background, and neurons affects stimulus detection.
• Stimulus detection is impaired if background noise is high relative to stimulus strength (low signal-to-noise ratio, SNR).
• SNR is a measure of the fidelity of signal transmission and detection by neurons and synapses.

Psychophysics and the Origins of Experimental Psychology

• Psychophysics: origins of psychology as an experimental science in the mid-19th century.
• Ernst Weber and Gustav Fechner explored how we detect sensory stimuli and differences between them (1834-1854).
• Weber's law: describes just noticeable differences.
• Fechner's law: the intensity of a sensation increases as the logarithm of an increase in energy.
• Steven’s law (Stanley Smith Stevens, 20th century): perceptual intensity increases as the n-th power of stimulus intensity.

Sensory Systems

• Sensory systems: structures containing receptor neurons, specialized sensory interneurons, and other projection layers of neurons specialized for detecting and processing particular types of sensory stimuli.
• Includes sensory organs (eye, ear) or sensory body systems (touch).
• Questions about sensory organs:
  • What information do they process?
  • How do they filter and transmit information?
  • How sensitive are they?
  • How do they adapt?

Signal Transduction in Receptor Cells

• Receptor cells transform stimulus energy into neural signals.
  

• Sensory receptor neurons:
  1. Specialized neurons that detect internal and external stimuli of a particular sensory modality.
  2. Input zone contains accessory structures, receptor molecules, and/or specialized ion channels instead of dendrites.
  3. Transform stimulus energy into neural signals (transduction) that are transmitted to sensory interneurons.
  4. Filter stimulus energy because they have a defined affinity and sensitivity range.

Classification of Sensory Systems

• Overview of sensory systems, modalities, and adequate stimuli (internal and external).
  

• Current research focuses on the enteric nervous system (ENS) and its role in neurodegenerative disorders and brain-gut disorders.
• ENS supports local muscle reflexes for gut motility.
• Vagus nerve (X) projects to enteric neurons and neuropods (enteroendocrine cells), providing CNS control of digestive functions.
• Sensory functions of the vagus nerve are critical for conscious perceptions and monitoring visceral functions.

Sensing External Stimuli

• Examples of receptor neurons and sensory interneurons in different sensory organs:
  • Taste receptors in the surface of the tongue.
  • Photoreceptors in the retina of the eye.
  • Hair cells in the inner ear.
  • Pacinian corpuscles in the skin.
  • Olfactory receptors in the olfactory epithelium of the nose.

Sensory Projections to Brain Areas

• Sensory projections to different brain areas are segregated (labeled lines).
• Functional specialization and segregation of brain areas allow the brain to discriminate between different information encoded in sensations.

Mechanoreceptors

• Mechanoreceptors mediate:
  • Touch and pain via diverse receptors in the skin and body
  • Posture control via proprioceptors in muscles and joints
  • Hearing via hair cells in the inner ear
  • Balance control via vestibular receptors in the vestibular apparatus
    *Each type of mechanoreceptor (stretch, vibration, pain, touch) has a distinct pathway to the brain.
• Different qualities of skin stimulation are communicated to distinct brain areas.

Sensitivity to Mechanical Stimulation

• The skin is very sensitive to mechanical stimulation.
• Several somatosensory systems encode touch via arcs and somatosensory pathways.
• Morphological details include encapsulated nerve endings and myelinated axons.
• Soma of skin receptors are located in the dorsal root (spinal) ganglia of the spinal cord.

Receptive Fields of Neurons

• Small receptive fields: Free nerve endings, Merkel’s disc, and Meissner’s corpuscle sense stimuli in small areas of the skin.
• Large receptive fields: Pacinian corpuscles and Ruffini’s endings innervate deeper layers of the skin and are sensitive to stimuli over larger areas.

Signal Transmission from Skin to Spinal Cord

• Pacinian corpuscles (detecting vibration and pressure) are unipolar cells that extend one branch of their axon to the skin and the other to the spinal cord.
• Afferent projections form the dorsal root (spinal) nerve, and the cell bodies are part of the dorsal root (spinal) ganglion.

Mechanically-Gated Ion Channels

• Vibration or pressure on the skin deforms the corpuscle and stretches the tip of the axon, opening mechanically-gated ion channels.
• Concentric layers of tissue (like an onion) around the axon tip amplify the signal.

Sensory Signal Transmission in Spiking Receptor Neurons

• Receptors respond to stimulation with a graded potential, the receptor potential.
• Spiking receptor neurons convert the graded receptor potential into action potentials for fast and long-distance transmission along the axon.

Sensitivity Differences in Mechanoreceptors

• Merkel’s disc and Meissner’s corpuscle sense innervate the surface of the skin and are sensitive to stimuli in small areas of the skin (small receptive fields).
• Pacinian corpuscles and Ruffini’s endings innervate deeper layers of the skin and are sensitive to stimuli over larger areas of the skin (large receptive fields).

Receptor Response: Tonic vs. Phasic

• Tonic receptors show a slow loss of response.
• Phasic receptors show a fast loss of response.

Receptor Response Properties

Sensory Information from Receptor Potential

• The four receptors transmit different information, such as initial contact, texture, slippage, and continuous contact.

Response Thresholds in Receptor Neurons

• Receptors respond to a stimulus within a limited range of stimulus intensities.
• The receptor response curve describes the sensitivity range of a receptor (e.g., low-threshold neuron responds to lower stimulus intensities than high-threshold neuron).
• Absolute threshold: lowest possible stimulus intensity detected or highest stimulus intensity that can be discriminated.
• Experimental threshold measurements use the method of constant stimuli, presenting stimuli of different intensities in random order.

Threshold Differences: Low vs. High Intensity Stimuli

• Low-threshold neurons respond with different spike rates to different low-intensity stimuli.
• High-threshold neurons do not respond to low-intensity stimuli.
• Low-threshold neurons reach maximal spike rates for different high-intensity stimuli.
• High-threshold neurons respond with different spike rates to different high-intensity stimuli.

Connecting Receptors, Brain, and Body

• Sensory pathways project to both subcortical and cortical brain areas and often have parallel streams.
• Information is filtered, combined, or enhanced as it passes from one layer to the next in serial order.
• Each layer has networks composed of input and output neurons, and many interneurons.
• Sensory signals are typically transmitted within a sensory pathway in a hierarchy of processing steps (labeled line principle).

Receptive Fields of Brain Neurons

• Receptive fields (RFs) can be mapped for neurons in different layers of a sensory pathway.
• Mapping involves recording from a neuron in the primary somatosensory cortex while touching a specific area on the body surface.
• The size, shape, and sensitivity within the receptive field are mapped.

Somatosensory Pathway

• Segregated projections to different areas of the brainstem, thalamus, and cortex (labeled-line principle).
• Spatial location of stimuli is preserved by separating projections from receptors in different locations.
• Connectivity involves top-down projections that modulate signal transmission via bottom-up connections between layers.

Cortical Encoding and Somatotopic Maps

• Adjacent regions on the body are generally encoded in adjacent regions in the cortex.
• The primary somatosensory cortex is located in the postcentral gyrus in the parietal lobe of the human brain (Brodmann areas 1, 2, and 3a,b).

Sensory Segregation in Cortical Somatotopic Maps

• Fast adapting signals (from Meissner’s and Pacinian corpuscles) and slow adapting signals (from Merkel’s disks and Ruffini’s endings) remain segregated in the cortex.

Sensory Maps in Vertebrates and Invertebrates

• Somatosensory map in the cortex of the star-nosed mole for the different tips of the star-shaped nose (touch organ).

Experience-Dependent Plastic Reorganisation of Cortical Maps

• Cortical maps can be reorganized due to experience.

Suppression of Sensory Input

• Receptors and the signals they convey need to be ‘switched off’ at times.
• Suppression often involves accessory organs that reduce the intensity or alter the stimulus before it reaches the receptor (e.g., eyelids, muscles in the middle ear).
• Neural top-down processes can selectively dampen sounds.

Rubber Hand Illusion

• The Rubber hand illusion demonstrates that touch can be sensed without a stimulus.

Summary

• Psychophysics, experimental psychology, and neuroscience investigate the relationship between sensation, perception, behavior, and neural activity in the brain.
• Receptor neurons are specialized to detect stimuli of particular sensory modalities, filter stimulus information, and transform stimulus energy to neural signals.
• Mechanoreceptors, such as the Pacinian corpuscle, transmit signals from the skin to the spinal cord.
• Receptor neurons can have different receptive fields and response thresholds, enabling filtering and coding of information.
• Receptors can adapt their threshold over time to optimize coding of the stimuli (phasic or tonic).
• The labeled line principle ensures that different receptors project to different areas of the brain.
• Serial processing of receptor signals is accomplished via transmission in sensory pathways with hierarchical layers.
• Somatotopic maps encode adjacent regions on the body in adjacent regions in the cortex.
• Experience-dependent plastic reorganization of cortical maps can occur.
• Sensory input can be suppressed with the help of accessory organs or through top-down processes.