Namaste, please help with the understanding of Atma is a witness or Sakshi. Reference: BG 9:18. In the materialistic world, the act of witnessing requires something (perhaps light) to enter something (receptors) of the witness (observer as witness). Since atma is indriyatheeta, how should one interpret the function of atma witnessing?
Witness or Sakshi is to be self Aware, or to bear witness to the one's own existence, which is nothing but to witness the infinite, Whole, Brahman. Because there is no Second entity to witness other than witnessing the self. It not a sensory awareness that is outward, like we have. This is what being conscious-of-self means. The Sanskrit term for "conscious of self" is called "Prajnanam Brahma". It was Aitareya Upanishad explained in detail in its 3rd part.
In the beginning all this verily was Atman only, one and without a second. There was nothing else that winked.
1 Who is He whom we worship, thinking: "This is the Self"? Which one is the Self? Is it He by whom one sees form, by whom one hears sound and by whom one tastes the sweet and the unsweet?
2 Is it the heart and the mind. It is consciousness, lordship, knowledge, wisdom, retentive power of mind, sense knowledge, steadfastness, though, thoughtfulness, sorrow, memory, concepts, purpose, life, desire, longing: all these are but various names of Consciousness (Prajnanam).
3 He is Brahman, He is Indra, He is Prajapati; He is all these gods; He is the five great elements-earth, air, akasa, water, light; He is all these small creatures and the others which are mixed; He is the origin-those born of an egg, of a womb, of sweat and of a sprout; He is horses, cows, human beings, elephants-whatever breathes here, whether moving on legs or flying in the air or unmoving. All this is guided by Consciousness, is supported by Consciousness. The basis is Consciousness. Consciousness is Brahman (Prajnanam Brahma).
4 He, having realised oneness with Pure Consciousness, soared from this world and having obtained all desires in yonder heavenly world, became immortal-yea, became immortal.
The same statement goes to Rudra, Taittirīya Saṃhitā 1.8.6, which says
“एक एव रुद्र न द्वितीयाय तस्थुर्” meaning “There is a Rudra without a second”.
Svetasvatara Upaniṣhad says the same:
एको हि रुद्रो न द्वितीयाय तस्थुर्य इमांल्लोकानीशत ईशनीभिः। प्रत्यङ्जनांस्तिष्ठति सञ्चुकोचान्तकाले संसृज्य विश्वा भुवनानि गोपाः॥ There is (हि) The One (एको ) Rudrā (रुद्रो) and none (न) other than He, none can make Him second (द्विती) in being (याय) that is in existence (तस्थु:र्य) among the worlds( इमां:ल्लोका), He is the authority (ईशते) by His own authority (ईशनीभिः)| In all worlds/dimensions (भुवनानि) is His convolution and projection and guardians (संसृज् + ज्य + गोपाः) in entirety (विश्वा), He is established (तिष्ठति) in all beings (हे जनाः) as the indweller (प्रत्यङ्); and all beings (भूत्वा), at the time of final dissolution (अन्त:काले), become/withdraw into Him (सञ्चुकोच) Svetasvatara Upaniṣhad 3.2
Chandogya Upaniṣhad says the same:
“from a single ball of clay, we can know every form made of clay, the difference in form is but the name (nāma-rūpa). In the beginning, was one being, without a second, or non-being, without a second; and from that various beings came to be. Just like bees make one honey from nectars of various flowers, yet the honey do not know from which tree or flower, in the same way, all beings begotten from One Being do not know their source” Please note: the word “Being” doesn’t mean a person nor an alien or animal, beings means “to be” or “to exist”. Existence can’t be described or iconified with in image/form within the frontier of vocabulary.
Chandogya Upaniṣhad 6.1.4/6.2.1/6.2.2/6.9.1/6.9.2
Taittirīya Saṃhitā says:
“He is the chariot, He is the rider in the chariot, who is the charioteer and He is the maker of chariot”TS4.5.2.
A similar analogy landed in Mahābhāratam which says:
“Thou art the fire upon which the sacrificial butter/ghee is poured. Thou art he who pours the ghee. Thou art he in honor of whom the ghee is poured, thou art the butter itself that is poured” MDP285.
Taittirīya Saṃhitā 4.5.2 / Mahabharatam Itihasa Drona Parva 285
Brhadāraṇyaka Upaniṣhad 3.9.4 associates the term Rudra with SELF:
These 11 types are divided into 10 vital Prāṇa that enter 5 karma-indrias and 5 jnana-indrias and the 11th is the Atman/self/mind.
The closing hymns of Śrī Rudram address Rudra as the interconnected prana of all beings (प्राणानां ग्रन्थि-रसि).
The same is found in Ramayana of Ṛṣi Vaśiṣṭha which says:
Rudra is the pure, spontaneous self-experience which is the one consciousness that dwells in all substances. It is the seed of all seeds, He is Prana, He is Atman, it is the essence of this world’s appearance, it is the greatest of actions. It is the cause of all causes and it is the essence in all beings, though in fact it does not cause anything nor is it the concept of being, and therefore cannot be conceived. ~ Yoga Vaśiṣṭham 6.1.36
First, you have committed a causal fallacy in your opening. You have equated the material act of perception with the 'perception' of Brahman. How men perceive this universe is limited to the senses and to the sensual universe - the realm of Maya. Brahman is beyond Maya, beyond the senses. You have been trained since you took up the body to perceive through the senses; you have taught yourself that there is an outside component that must be perceived by your brain.
Swami Vivekananda says (Complete Works of Swami Vivekananda, vol 7, 'Inspired Talks, Wednesday July 17,1895' - https://advaitaashrama.org/cw/):
We cannot say positively what differentiation is. All that we see and feel about things is pure and simple existence, "isness". All else is in us. Being is the only positive proof of we have of anything...Shankara says again, perception is the last proof of existence. It is self-effulgent and self-conscious, because to go beyond the senses we should still need perception. Perception is independent of the senses, of all instruments, unconditioned. There can be no perception without consciousness; perception has self-luminosity, which in a lesser degree is called consciousness. Not one act of perception can be unconscious, in fact, consciousness is the nature of perception. Existence and perception are one thing, not two things joined together. That which needs no cause is infinite; so as perception is the last proof of itself, it is eternal. It is always subjective; perception itself is its own perceiver. Perception is not in the mind, but perception brings mind. It is absolute, the only knower, so perception is really the Atman. Perception itself perceives, but the Atman cannot be a "knower", because a "knower" becomes such by the action of knowledge; but, Shankara says, "This Atman is not I", because the consciousness "I am" (Aham) is not in the Atman. We are but the reflections of that Atman; and Atman and Brahman are one.
Further, as Sri Krishna says in Gita 9.4-6 (Swami Nikhilananda translator):
By Me, in My unmanifested form, are all things in this universe pervaded. All beings exist in Me, but I do not exist in them.
And yet the beings do not dwell in Me--behold, that is My divine mystery. My Spirit, which is the support of all beings and the source of all things does not dwell in them.
As the mighty wind blowing everywhere ever exists in aksha, know that in the same manner all beings rest in Me.
When you are in the desert and you see a mirage, is the mirage real? Do you exist in the mirage, or does it only exist within your own perception? Are you not the "witness" to the mirage? Maya is a the mirage, the perceiver is the Atman.
As Ramanaya Maharishi said, the only question to ask is "Who am I?"