12.2 Applications and skills

12.2.3 Variations in sensory perception

Other organisms detect stimuli that humans cannot, and among humans there is individual variation in the types and concentration of sensory receptors.

Application: Olfaction involves many chemical receptors

  • Our sense of smell involves chemoreceptor cells in the nasal epithelium that synapse with the olfactory nerve in a region of the brain called the olfactory bulb.

Figure 12.2.3a - Anatomy of receptor cells and the olfactory bulbFigure 12.2.3a - Anatomy of receptor cells and the olfactory bulb

  • Each of the smell receptor cells responds to a specific chemical and when combined in various concentrations, it is estimated that a human can detect between 103 and 109 different smells.
  • The ability to distinguish smells, and the threshold concentrations at which a smell can be detected, vary among individuals.
  • In general, women have a better sense of smell than men, and many women report a heightened sense of smell during pregnancy, indicating that there might be a hormonal cause for the difference.

Application: Colour blindness is a variant of trichromatic vision

  • Colour blindness is a sex-linked genetic condition that results in a defect in the gene that codes for a pigment necessary for either red or green cones in the retina.
  • There are many variations of colour vision deficiency but since all forms are determined by recessive genes located on the X-chromosome, colour blindness is more common in men than in women.

Figure 12.2.3b – Colour blindness test

Figure 12.2.3b – Colour blindness test
Sample plates from a complete colour blindness test. People with normal vision should be able to distinguish the numbers 12, 2, 42, 74 and 6 from these plates.

  • Defective colour vision can also be caused by macular degeneration due to ageing, or diseases such as cataracts or glaucoma.

Activity: Trichromatic vs tetrachromatic vision

Figure 12.2.3c shows the range of wavelengths detected by each type of retinal cone in three different species.

12.2.3c – Absorption spectraFigure 12.2.3c – Comparison of absorption spectra in the cones of humans, bees and birds

  1. State which species has the greatest range of colour vision.
  2. Compare the spectra for the three species.
  3. Recent evidence suggests that some humans are tetrachromatic. Discuss the evidence that tetrachromacy increases colour perception.

Application: Use of cochlear implants by deaf patients

  • Hearing aids amplify sounds, but they cannot restore hearing to patients with damaged sensory hair cells.
  • When deafness is caused by damage to hair cells, an implant can be used to bypass the damaged cells and transmit electrical impulses to the auditory nerve.
  • Instead of converting pressure changes into electrical signals, a cochlear implant stimulates the auditory nerve directly by creating an electrical field inside the cochlea.
  • Cochlear implants are a great improvement on hearing aids, but if the auditory nerve is damaged, they are not useful.

cochlear implant

Figure 12.2.3d – A cochlear implant

  1. The external portion of the implant consists of a microphone and speech processor, as well as a transmitter. The speech processor contains a small computer that divides the sounds into groups based on wave frequency.
  2. Radio signals from the transmitter reach the receiver. The transmitter and receiver are attached across the skin with a magnet.
  3. A small computer in the receiver converts radio signals into electrical signals that are transmitted through the stimulator, which is embedded in the skull.
  4. The electrode array extends out of the end of the stimulator tube. The array is made of an electrically-conductive and flexible material that winds through the cochlea. As electricity is passed through the electrodes, the auditory nerve is stimulated.

12.2.3eFigure 12.2.3e – Dog scent
Dogs are much better than humans at isolating scents, but their colour vision is limited. Like most mammals, they have only two types of photosensitive cones, one whose sensitivity peaks in the blue range and one in the yellow range.

12.2.3fFigure 12.2.3f – UV pattern

TOK

Other organisms can detect stimuli that humans cannot. For example, some pollinators (e.g. bees and hummingbirds) detect electromagnetic radiation in the non-visible range – they can ‘see’ UV light. As a consequence, they might perceive a flower as patterned when we perceive it as plain. To what extent is what we perceive merely an individual construction of reality?

12.2.3gFigure 12.1.3g – Cochlea
Micrograph of human cochlea showing hair cells. Damage to the cochlea can be caused by exposure to loud noise, as well as congenital and degenerative diseases.

Nature of Science

Science is the basis for technological development: the discovery that electrical stimulation in the auditory system can create a perception of sound resulted in the development of electrical hearing aids and ultimately cochlear implants.

Further reading

  • On the link between hormones and olfactory sensitivity: Tory Rodriguez (2013) ‘Fertile women have a heightened sense of smell’, Scientific American 24(4).
  • On tetrachromatic vision in humans: Greenwood, Veronique (2012) ‘The humans with superhuman vision’, Discover Magazine. Accessed at: http://discovermagazine.com/2012/jul-aug/06-humans-with-super-human-vision

Course links

The genes responsible for red-green colour blindness are located on the X-chromosome. Review the principles of chromosomal inheritance and sex linkage in 3.1.4.